Of course there are discrepancies over whether or not Sulla was a good man and/or a good ruler - he was a complex man who did both good and bad things as an individual and as a ruler.

For starters, he was a patrician with very conservative traditional tendencies. He strongly believed that Rome should be run the way it always had been run. This is important to remember.

While Sulla was serving his term as duly elected consul, the Senate gave him command of the war against King Mithridates, who had invaded some Roman provinces out to the East. This was standard practice in the day: consuls led Rome's armies into war. As Sulla was leaving Italy with his army, he got news that Gaius Marius back in Rome had bypassed the Senate and got the People's Assembly to revoke Sulla's command and give it to Marius - who had previously been a great general and consul and was a true hero of the people, but who was now a broken-down old man who wouldn't let go of his glory days. Sulla marched back to Rome and sorted that out, but it left a very bad taste in his mouth.

While he was away at war, another rival of his back in Rome went to the People's Assembly and got Sulla declared an enemy of the people. Any laws he had passed as consul were struck from the books (regardless of whether they were good or bad laws), his house was destroyed, and his family had to flee Rome. And he was yet again relieved of his command against Mithridates, and a new army under a new general was sent out east to take over.

Naturally, Sulla was mightily pissed. He hadn't actually done anything wrong at this stage, except make some political enemies in Rome - just like every one of his peers had done. He didn't really deserve the treatment he was getting at this time; it was just that Gaius Marius (now dead of old age) had set a precedent and showed people how to bypass the Senate to get their own way. So, Sulla was the target of every unscrupulous ambitious man just because he happened to be in charge of a high-profile and profitable war.

Long story short: Sulla marched on Rome to get what he was legally entitled to, fought a civil war, and won.

This is when he got the Senate to vote him in as dictator.

And, this is when things get murky. Because Sulla did some very bad things as dictator - but for what he saw as good reasons. His goal was to fix a broken Rome. The Senate should be in power, not the People's Assembly. Random ambitious people should not be stripping legally elected and appointed consuls from their command of Rome's armies. And so on. Rome's government was broken, and Sulla intended to fix it.

However... he started the process of fixing it by arranging for hundreds of people to be proscribed as "enemies of the state" (which often meant "enemies of Sulla"). These people were murdered or exiled, and their assets were confiscated by the Senate. Part of the motivation for these proscriptions was purely fiscal: the Treasury was empty, and there was no other way to get the money that Rome needed to survive (there weren't really any taxes in Rome, just tributes from provinces). In some cases, the proscribed man was allowed to flee into exile: the important thing was to acquire his assets for the Treasury. Of course, there were some people Sulla wanted dead regardless, but money was also a motive.

Having killed off the troublemakers and put the fear of Sulla into everyone else (nearly 2,000 people died during those proscriptions), and filled the Treasury, Sulla set about fixing Rome's government. He produced Rome's first written constitution, laying out the powers of the Senate and the People's Assembly, and formalising the unwritten rules about the "cursus honorum" (the "path of honour"), which was the sequence of elected offices a man should progress through during his political career: eligible to be elected as quaestor at 30; eligible for aedile at 36; eligible for praetor at 38; eligible for consul at 40 (with plebeians having to be two years old than these ages set for patricians - because patricians, as the ruling class of Rome, should be privileged over plebeians). Also, a man had to wait 10 years before he could be elected as consul for a second time. Basically, he restored Rome's traditional constitution, to make Rome's government more stable and less vulnerable to the types of things that had happened to him: ambitious upstarts removing legally elected consuls from their office. Ironically, under his new constitution, Sulla was not only preventing people from following Marius's example and becoming consul seven times (unheard of before Marius), but also ensuring that no one man ever dominated Rome's government - like Sulla himself. Sulla was preventing future Sullas!

Then, having been the first Roman to conquer Rome by military force, having killed hundreds of Romans of the ruling classes, having confiscated enough wealth to fill Rome's Treasury, having re-written Rome's constitution to make Rome a stable and traditionally governed city again, having had total absolute power over Rome for two years... Sulla stepped down voluntarily as dictator and resigned to the country. He left Rome in a scandalous parade of prostitutes, dancers, and actors (Shock! Horror!) - complete with his younger male lover, an actor named Metrobius, with whom he lived out the rest of his days (only another year or so).

Was he good or bad? Noone can decide! That's why you'll never get a clear answer on this question. Sulla was a very complex man.