I feel as if I need to pre-face most of his life before actually answering this question in full.

To support Shaikoten's reasons of financial desperation, and as I wrote in a P.M. to Brosen87, Arnold came from a very wealthy family, that wealth unfortunately dwindled sometime in 1753-55, when Arnold's siblings, Absolom and Mary, died a few years apart - this drained the family's fortune, and Arnold's father unfortunately fell in to a serious alcoholism. This led to Arnold being unable to be privately schooled, and not becoming a merchant, but instead becoming a druggist. Anyway, what this all lead to was him volunteering in the British army during the French/Indian war, and he only served for a mere 2 weeks before returning home. After this, his mother passed away, and his father shortly thereafter - All in all, Arnold wasn't really well off, and I believe this partly contributed to his reasons for defecting later on.

In the Revolution, as Shaikoten said, Arnold had some great achievements - He was lauded as a great General, and everything Shaikoten said about him is more or less correct, a Patriot if you've ever seen one. After suffering a leg wound in combat, he had spent an alarming amount of money, even going so far as to drop over £1,000 ($78,000 in 2013) on a pipe of wine (Source: Patriots, p. 500). This lavish spending led to his fortune decreasing quite rapidly, but he was compensating for his debilitating leg wound that he suffered during the Invasion of Quebec, where General Montgomery was killed, which made him unfit for any active field command, rather putting him behind a desk. Just a couple of months before his treachery, he had been court-martialed, and narrowly avoided conviction, which led to him falling out of favor with Washington himself, who was at this point 'His Excellency, the Commander in Chief', but Washington respected him enough as a soldier to more or less just give him a slap on the wrist. Arnold had been given command of West Point in 1780 and had moved in just across the river with his wife, Peggy. She had also given him a child not long before this, and it seemed as if Arnold was finally happy and content.

Only earlier in the conflict, Arnold felt he had been seriously wronged. He had been passed over for the promotion of Major General (6 Junior Officers had been promoted ahead of him, Benjamin Lincoln, Thomas Mifflin, Arthur St. Clair, William Alexander, Lord Stirling, and Adam Stephen). Washington took up Arnold's case and wrote, personally, to members of congress to try and sway Arnold's case, and pleaded with them not to make politically motivated promotions, as he felt it'd negatively impact other officers. He actually wrote a letter to Arnold on the 3rd of April, 1777, stating his surprise over not seeing Arnold's name in the list of new Major Generals, he urged him not to take any hasty action as he was sure it was simply a mistake. It wasn't.

Arnold was again wounded in battle at the Battle of Ridgefield, again in the leg. This, he believed himself, led to the proper promotion of Major General, although he held no seniority over other Major Generals, something he bitterly despised. He submitted his official resignation in July of that same year, but this was soon withdrawn after Washington urged him to do so. Arnold's actions at Saratoga, especially having more than just a disagreement, more a full-fledged shouting match with General Gates, which ultimately led to Gates demanding Arnold leave the battlefield, were critical. However, in the second Battle of Saratoga (October 7th), Arnold returned to field duty despite his commanders wish, and was wounded for a third time in his left leg (Not a lucky streak), and Congress decided to restore his command seniority, but he felt as if it was sympathy, not his actions, that decided this 'restoration'. It left a bad taste in his mouth, and after being contacted in 1780 by John André for possible negotiations of defecting his command to Britain, he was probably given an offer he couldn't refuse.

Given his prior 'insults' by the Continental Congress, he felt as if he was being mistreated and he felt as if he didn't receive the recognition he deserved. He had certainly been a valuable asset to the American cause thus far, why shouldn't he be treated as such? Taking Arnold's prior loans and funds put in to the Continental Militia and his general financial situation in to account, as well as his marriage to Peggy Shippen, the daughter of a very well-connected Loyalist family, £6,000 was a lot of money, on top of an annual £360 for his Brigadier General commission. I think this, as well as the fame he envisioned himself gaining with the British Army (Something he had desperately hoped would come out of his campaign in Canada), was enough to sway him.

His own greed and rash decisions and ill temper had led to this decision, and his misinterpretation of situations certainly didn't help either. He had taken insults where none were given, and he had given insults where none were necessary, and he had tried to resign his command twice, and had probably suffered both mentally and physically from the 3 leg wounds and his young wife's apparent mental illness, it would've definitely taken it's toll on his mental health. This would also explain why he apparently left his wife alone with their child at Fort West Point, it could also be explained away by the fact that Washington was coming for a surprise visit, something Arnold most likely wasn't prepared for.

Unfortunately for Arnold, the British army didn't really work out for him. He was promised a lavish £20,000 for the succession of his plot, but since it had failed, he was paid a 'measly' £6,400, and a pension of £360 yearly. Later, as a British Brigadier General, he had led some 1,500-1,600 men in to Virginia and captured Richmond amongst others, but unfortunately the Southern militia turned out and knocked them back, but the 'Southern Civil War' is an entirely different story for another day, and he left for England in 1781 to consult with Lord George Germain about the current war efforts, later dying in London in 1801. John André, the man who orchestrated Arnold's betrayal, was hanged as a common spy after three Patriots, one dressed in a Redcoat's uniform (Leading André to confide his purpose to him), stopped him on the outskirts of Tarrytown, and confiscated his incriminating documents.

As I was brought up in Arnold's native Connecticut, there were quite a number of discussions on the man. Militarily he was one of the best performing American officers during the revolution. He had a quick wit, inventive tactical thinking, and was fiercely dedicated. He spent his personal fortune on raising and training troops for the Continental forces because he believed that British rule and taxation were making his shipping endeavors unprofitable. Simply put, the man was fiercely patriotic.

What led him to his fall was a combination of pride and financial desperation. He was keen for advancement within the Continental forces, and although he was well liked by Washington he had suffered two injuries to his leg which made him largely unfit for field service, instead being relegated to defensive roles, like those of Philadelphia and later West Point. Especially in Philadelphia he spent lavishly as a member of high society, while grumbling about the performance of Congress, their inability to properly feed the troops, and their unwillingness to accept the Carlisle Peach Commission's offer of American autonomy within the British Empire. Consequentially, running his mouth had made him no shortage of enemies within both the Continental Army and Congress, and he was going broke.

It's unclear to what extent an ideological belief that America would be better off back under British rule played in Benedict's decision, but it seems very clear that money and commission in the British army was no small factor to him. Upon the discovery of his plot, the money portion of the deal was played up quite a bit by the American propagandists, likening him to Judas. Essentially he offered to sell out West Point to the British for 10,000 pounds and a commission, and while in negotiations the British agent John Andre was captured with plans for the defenses of the fort on his person, and Arnold's role in the plot was uncovered.

Arnold ran, escaped, and defected and generally got treated even more poorly by the British than he thought he was being treated by the Americans. Since the plot failed he only got paid in part, and no one trusted him with command, since he was a slippery traitor and he was a cripple besides. He died relatively impoverished and in obscurity.

As far as his legacy is concerned, the American papers of the time as well as the prominent political figures were quick to pile their derision on him. He was held up as one of the earliest examples of what a BAD American was; his lack of perseverance and greed, as opposed to the selflessness of other revolutionaries like Connecticut's state hero Nathan Hale of, "My only regret is that I have but one life to give for my country" fame. The reason his name is synonymous with traitor in American circles is because he was the first prominent traitor that the country could identify with. No different than, say, Brutus for Rome or Judas for Christianity.

There are lots of questions about him on this subreddit: http://www.reddit.com/r/AskHistorians/search?q=Benedict+Arnold&restrict_sr=on

You mentioned that he was fiercely patriotic? Wasn't he, as well as other Revolutionaries, traitors to the British Crown? They were rebels, not too much different from the Confederates, but they are patriots simply because they won?