I literally see no difference in killing a person just for fun and killing someone in self-defense. It's the same act. Just done for different reasons.

All are "acceptable" in my eyes. Just as all are "unacceptable". It's really neither. It just is. I can look at what caused the killing, and then decide if I personally would've gone through with it, but really, any example can be extreme or not depending on your opinions.

As I said, I just weigh pros/cons. Killing in self-defense, AFAIK, would be more defendable in law. At least, I'm pretty sure it is. I haven't done much research into killing people, so I apologize.

The best way I can explain it is this:

There is a guy (lets call him X) that you are told to assassinate. Now, X is a pretty smart dude. Lets say he's the top in some respected scientific field. Would you kill him? Probably not. There's no reason. The guy doesn't seem to be doing any harm (to you or anyone you care about), and he overall seems to be advancing his field. That and you don't have any external motivator. Is it morally right? Pretty much everyone would say no. I'd say that it's neither (or subjective).

Now add in money. Say 10 million dollars. Would you do it now? Some people yes, some people no. Is it now morally right? Most people would say no. I'd say the same (it's neither/subjective).

Now lets say the guy is a serial killer. So you get 10 mil if you kill him, he's a serial killer, but he's also a respected scientist advancing his field. Would you kill him? I'd guess a lot more people would say yes. Is it morally right? Here's where people get stumped. The reasoning would go: "well he's a good scientist, but he's pretty evil. But killing is wrong." So some people again would say no, and others yes. It really all depends. I'd say the exact same thing (neither/subjective).

Now lets add in complete immunity. Legally, you're free to kill him. No jail time, and the government is supporting you. Would you kill him? I'd bet most people would probably say yes (unless they have a problem with killing specifically). Is it morally right? I honestly don't know what people would say. Probably yes. Maybe some nos. I'd still say the same (neither/subjective).

Now lets say he tried to kill you, and you killed him in self-defense. Is that morally right? At this point, there's a whole range of different opinions on it.

I personally think that you've just earned 10 million dollars, killed a serial killer (saved some possibly good lives) and have complete immunity. You've also just protected yourself. Overall it's a pretty good deal. I don't see that as wrong or right. Just beneficial.