Absolutely not. That would be murder! To a modern audience the distinction perhaps seems to be a slight one, but the underlying purpose of a duel revolved around honor, and shooting your opponent outside of the rules of the duel would be a very dishonorable thing to do. That isn't to say that plenty of duelists wouldn't have loved to drill one into the back of their opponent, but while even going into the 19th century (or 20th in some places such as Wilhelmine Germany) a jury would usually be willing to not convict a duelist, or at most give a lesser conviction of manslaughter instead of murder, a killing that didn't follow the 'code' would not give you the same protections. One of the reasons for seconds was to act as witnesses to the encounter and also to ensure proper conduct, and even intercede if this was not the case. As long as things went by the book, they would of course refuse to testify to incriminate the participants, but you can be sure they would publicize the ill-conduct of one party were they to not follow the code.

In the Anglo-American tradition, I would add, aiming at all, or even practicing at dueling style shooting, was quite frowned upon. The style of snapping to and immediately shooting was de rigueur, and failure to comply could result in great condemnation - such as with Andrew Jackson, who killed Charles Dickinson with a carefully aimed shot after letting Dickinson take his (quick, snap-)shot first - which, it should be noted, hit Jackson. This wasn't explicitly against "the rules", but very frowned upon. Jackson did most likely violate the code however, as he had a misfire, recocked, and fired again successfully. Dickinson's second rightfully could have interceded, as Jackson's misfire counted a shot. He was never charged for the duel, but many people considered him a murder for it - not for the duel itself, but how he had gone about it.

Another thing to note is the practice of deloping, which was an intentional discharge of the weapon with no intent to hit, often either into the ground, or directly into the air. Most codes considered this to be very improper. As noted in the Irish Code Duello, perhaps the most famous of duelling codes:

No dumb shooting or firing in the air is admissible in any case. The challenger ought not to have challenged without receiving offense; and the challenged ought, if he gave offense, to have made an apology before he came on the ground; therefore, children's play must be dishonorable on one side or the other, and is accordingly prohibited.

It still happened though. Again to illustrate with a famous incident, in the Hamilton-Burr duel, whether or not Hamilton deloped his fire is still a matter of contention. He certainly fired early and high, but whether it was intentional or a slip of the finger we will never know for certain, as Burr didn't care one way or the other and shot him dead, demonstrating, perhaps, why if you intend to delope you should maybe just apologize.

So anyways, the point of these digressions is that while to shoot your opponent before the duel has commenced would be a big "no-no", there were still ways in which one duelist would essentially be cutting down a defenseless opponent.

Sources: I would probably recommend "A Matter of Honour" by Martyn Beardsley if you're interested in dueling with pistols. I've written a few other things on duelling before, and you can find them here. Includes a few more book recommendations.

Duelling was a matter for gentlemen wishing to maintain their honour in society. So they had to honourably abide by the rules or be pariahs. Natch, anywhere you don't have Society or where the individual is willing to flaut the law, mere murder may happen. But in that case you might as well shoot him in the street.

Now we should note that the duelling codes vary greatly by time and place. In the 1600s and early 1700s much more went on, normally by sword, and the meetings were often fiercely homicidal. In France, the seconds (and you might bring a handful), would fight each other, too.

The development of the pistol duel normally toned down the bloodthirstiness. The encounter was at a little distance, and it did not require great skill. In fact, luck was encouraged. Even when better pistols were developed, duelling pistols remained smooth-bore, flintlock (which missed fire about a sixth of the time), and without sights.

For all that, English gentlemen of the late Georgian period did practise shooting with their duelling pistols, at a place called Manton's. Going there in the evening with some friends was one entertainment possibility. A good shot could "snuff candles" at duelling ranges, with a snap shot, which sounds to me like modern point-shooting.

At Project Gutenberg, you can pick up an Irish code, bound with Gov. Wilson's code of fifty years later, and compare the greater emphasis on making up a quarrel. This is in harsh contrast to the 1840s Heidelberg student code in Howitt, where fights were picked on trivial grounds and apologizing was craven. But they always fought saber with protective gear to prevent fatalities. Duels between students and soldiers were run quite differently.