"Thou shalt not kill."
- Exodus 20:13 KJV (1769)
Taking someone's life's against the Law of Moses (The Ten Commandments). Nonetheless killing in self-defense's not punished by religion. At the same time, killing in time of war's not punished by religion or society. It all depends who's killing whom.
"Thou shalt try real hard not to kill anyone, unless, of course, they pray to a different invisible avenger than the one you pray to."
- George Carlin (1937-2008), November 17, 2001
In the case of capital punishment, societies have different opinions. Those who oppose would refer to the Law of Moses. On the other side of the coin, those in favor would say, "an eye for an eye, a tooth for a tooth." Extending this latter idea, it'd clearly be "a life for a life," where taking someone's life makes lots of sense if this person had killed at least one person intentionally. For the record, I support it as much as it may go against the Laws of Moses.
In the case of abortion, I'm against abortion as it's the selfish and coward murder of a child. Pro-choice's merely child-murder and those who support it are imbeciles. Further more, there's nothing worse than one's mother telling her son/daughter that she'd wanted to abort him/her. In such scenario, that child might be lucky to be alive -- my case as I cheated death for the first time (http://project05952381.blogspot.com/2010/11/cheating-death-on-borrowed-time.html). My sister (two years younger than I, age 37 if she'd been given a chance to live) and other siblings whom I was never aware about weren't as lucky. As much as I hate to admit, I lost a child this way too. That's enough for me to say.
I don't care to have an opinion when it comes to killing to get rid of someone (especially other criminals). Of course, I don't have an opinion as long as it's not a child (worse than abortion) or to leave no innocent witnesses behind during other kind of crimes (becoming a candidate for capital punishment).
In all, killing may simply be a sin/crime if killing the "wrong" person in the "wrong" circumstances. After all, we've all wanted to beat someone to death, shred someone to pieces and/or simply kill someone who's done us wrong. Of course as sinners, we always find a way to excuse ourselves and still go to church the very next Sunday.