To end a creature's life, usually through violence. Depending on who or what you kill, it may or may not be a crime. It's a crime to kill most human beings (unless you killed them to keep them from killing you or another person -- that's what self-defense is all about), but it is not a crime to kill an insect. Some people become killers on purpose, some by negligence, some by accident.

Also, to destroy the effectiveness of a group, organization, event, or object. You can kill a program, legislation, an advertisement, a keg party, and many other things. You can even kill love, but there's far too little of that in the world, so I don't recommend it.

I'd sorta like to tell you what it feels like to kill someone, but I can't, because I haven't. I'd like to tell you what other people say it's like to kill someone, but I won't because it's irrelevant. Everyone who kills someone feels something different -- some people enjoy it; some people seek out the opportunity to kill again; some people hate it; some people laugh; some people weep; some people swear off violence; some people become religious; some people give up religion; some people keep it a secret; some people confess; some people commit suicide. There are as many different reactions as there are killers on the planet.

As for the people on the other side of the gun/knife/automobile/blunt object... they just die.