I don't get it either.
Half the time, being "generic evil guy" in a game that allows for "good or evil" just boils down to "Hey! You can steal, murder and disregard collateral damage! EASY MODE AWAY (as long as you're cool with fighting off guards of some kind, who are tough at first, but eventually become a total joke)!"
It's boring and not all that interesting most of the time.
I prefer to play the good side. And the main reason is this:
TEMPTATION.
I play a hero in Mass Effect. And yet, I take a fair amount of renegade options. Why? Because they tempt me. There are plenty of times where the payoff for doing the "bad" thing will help me out quite well somehow, if only I can ignore my conscience.
Being good always leaves you open to taking the quick and easy way to success, and it's always there. It makes things WAY more interesting to me. On the other hand, if you're evil? It's VERY hard to "tempt" you to do good. Why would you? Aside from making yourself look good to people, there's next to no reason to do the right thing over the wrong thing if you're cool with doing evil most of the time.
The EPITOME of this is with Vampire the Masquerade (the tabletop game, not the video game).
Whenever I look for stuff online about it, I see SO many goddamn topics about "Oh hey, we just started a Sabbat Campaign and it's SO much fun! We lit fire to a mall and killed like...20 mortals and one guy drained another one dry and beat a hunter to death with the body!

" (ok maybe that's a bit of an exaggeration, but there are WAY more "I started an awesome sabbat campaign" topics than other faction campaigns).
To those who don't play, the Sabbat are the generic "Evil" faction. They embrace their monstrous nature and gleefully hurt humans and act like generic bad guys about 90% of the time. Which TOTALLY negates the entire POINT of Vampire the Masquerade, which is the slow, steady decline of your morality in the face of your monstrous nature, and whether you choose to fight it, or eventually succumb to your nature. STARTING in the Sabbat is basically saying "oh hey, I don't care about the central conflict. I just wanna light stuff on fire because why not!".
And this is coming from a player who is playing IN a Sabbat campaign now (Although I'm an Independent/No-Faction who was DRAGGED into the conflict because the Sabbat rule the city I'm hiding out in and they're basically planning on killing me if I don't help them resolve a major conflict, which has the potential to destroy the entire city). I'm a highly Moral character who is trying to keep the insanity of my two teammates in check and juggle my distaste of the Sabbat with the fact that if I don't help them, things could get even worse for the city (and they'll kill me).
And then there are my two partner players. One is a Mad Scientist who is only interesting because she finds all sorts of creative evil experiments to do, role plays them SUPER amusingly, and also has to contend with the fact that if her morality slips too much she will turn into a rabid animal, so she HAS to try to limit her madness. The other? A typical "Imma gonna kill ALL THE THINGS and have NO restraint!" street soldier lady.
The only reason that last character is interesting in the SLIGHTEST is because our Storyteller is REALLY good. He's made her desire for unrestrained violence a WEAKNESS and she's learning to try to control herself (she ROYALLY fucked up something in our last session. The ST said "Your character is prone to unrestrained violence, right? Roll for willpower. ...Oh, you failed? In a fit of revenge, you kill the person you were ordered to take alive." Cue the city falling into civil war). Plus, he's actually made the Sabbat more than just unrestrained monsters. They plot, they scheme, and they keep both tempting me AND making it clear that the only reason I'm not dead is the fact I'm being useful to them. (I shudder to think of what will happen if/when they uncover my character's Dark Past, which includes killing a VERY high ranking Sabbat member one time...).
I get the feeling most storytellers wouldn't be able to pull that off, or even want to. I keep seeing topics about how much fun they're having just going on creative massacres instead of the desperate (but fun) struggles against forces beyond our control, as well as our morality and personality flaws.