There's different reasons to pull your punches, depending on who you're talking about. Here's the breakdown.
Killing civilians: This is a big no-no. Unless you are completely crazy, you can threaten them, you can intimidate them, you can menace them or take their stuff, but you don't kill them. Oh, you want them to think that you could, because your threats will ring pretty hollow otherwise. But using your powers already escalates the severity of your crimes, and a dead stockbroker results in a very unsympathetic jury.
Killing cops: Also a big no-no, no matter how annoying they are. Cops are a fraternity. If you kill one of them, they will all come after you.
Killing supers: This is the biggest prohibition you need to worry about. You might think "but Mr. Big, if I kill my heroic nemesis, my problems are over!" That's a lie that noob villains tell themselves. If you kill your heroic nemesis, your problems have just begun.
- First, most importantly, you just killed a guy! What did he do to you to deserve that? If it was self-defense, and you picked a hero as your nemesis who you knew was likely to use lethal force, that's your mistake. Go back and re-read the introduction. If not, then you're still a terrible person. The best heroes are the single biggest source of entertainment for a good villain. If your hero isn't giving you the time of your life, one of you is boring. Find out which, and fix it.
- Second, unless you killed the last hero on earth, there will be more. And they will come for you. You think cops are a close-knit group? Superheroes are tighter than spandex.
- Third, unless you are some sort of world-breaking badass, odds are excellent that you'll fail anyway.
It's time for a biology lesson, kids!
All superpowers emerge from a single source, as far as anyone's ever been able to prove. Some people call this a "mutation". The truth is really a lot scarier, but I'm not going to talk about that here. The important thing is that it happens in stages.
Stage 0: this is a normal, mundane, factory-sticker human being. Some supers, like the vampire variants, can change this, but for the most part, if you're born at zero, that's where you stay.
Stage 1: this is "latent". You have the potential to develop powers. This stage lasts between one to three weeks after you're born, depending on whether you're a Child of Eve or a Child of Lilith. Not sure which one of these you are? It doesn't really matter, but basically you're a Lilith if you're an ugly monster and an Eve otherwise.
Stage 2: this is "potentiated". You're stronger, faster, smarter, or all of the above, than a regular human of your age and upbringing. Most of those brooding vigilantes of the night will be at this level, because they can't fly or shoot lasers or anything. They're more of what they already were, but they don't have actual superpowers yet. Their healing factor is pretty good but not amazing.
Stage 3 and above: this is where it's at. You have some specific power. You have a healing factor which is pretty impressive. If you're reading this guide, this should be the minimum level of power you possess. If not, sorry, but you're going to get curb-stomped by a real hero.
How do you go from one stage to the next? Stage 1 to 2 happens automatically after birth. Stage transitions after that happen when the body is subjected to life-threatening injury. The mutation will rapidly evolve to keep you alive, and to deal with the original problem.
But here's the thing. All supers at stage 2 have a healing factor. It just gets stronger as they go up in stages. Which means: if you badly hurt a super, and it's not enough to kill him, you just made him tougher.
I cannot emphasize how central this is to the rules of villainy. You don't want a tougher hero, do you? Of course not! So don't do anything that would cause it to happen!
Mr. Big's words of wisdom: Shoot to stun, have your fun. Shoot to harm, you've bought the farm.