No… I’ve never killed in my lucid dreams… It seems to real to me and just wrong. 
Hi
In my non-LDs i am sometimes violent (not that often,but from time to time…).
Never thought of being violent in a LD,i have better things to do when i am lucid 
If somebody would try to attack me in a LD i would perhaps strike back.But not in a simple fist-fight,i’d throw energy balls at him that make him freeze or something… 
Traumgänger
Last night’s dreams were very violent again. I must have killed 4-5 people with knives and I was stabbed several times, and it HURT. After I became lucid I got into two fights and used excess violence.
slaps self
Last night in a LD I hit my computer teacher in the head with a concrete column. She was at my house, which really pissed me off.
EDIT: Woohoo! 50th post!
I have had one scifidream where i was a captain of somekind of scifi-troops. There was some war going on and i had to shoot and kill about 10 persons. It didnt felt so bad, coz the dream was very videogamelike. And after all, there was a war going on - kill or get killed. Well… irl i couldnt kill anyone, not even in a war.
But then theres another dream, a lucid one. Actually my first real lucid dream where i was sitting on a bus and i heard annoying voice from somewhere in the back of the bus. The voice was insulting me so i tought “this is my dream and theres noone here who can insult me”. So i picked up a gun from my pocket, turned around and shot. The voice who insulted me belonged to a little boy who was forced to insult me. He didnt want to do it, he just had to do it.
And i felt so guilty after waking up. Why had i done such thing? I wouldnt do anything like that in real life. Even now i feel guilty when i think about it.
I have read that the actions you do in real lifes, will have consequences in your dreams, and the other way. So I am sceptical to commiting acts of violence in dreams, because I want to be a good person in waking life, and I fear that these actions can have bad influence at the waking-life me. 
Once when i was in a lucid dream i decided to try out some magic. so i found the closest person (someone i don’t remember seeing before in real life) and i focused on making his head explode. it did work and i felt kinda bad about it in the dream but upon waking up it didn’t bother me too much. plus i got to use magic for the first time in a lucid dream. 
I hold the exact same opinion as LucidityX1000, and besides there’s better things to try than acts of evil.
In a recent LD I was annoying a man going about his business walking down the street, getting in his way, and then I felt sorry because I thought of myself as invisible and I could have really freaked him out.
Have some pity for your dream characters! Maybe next time you ask them for the meaning of life, you might just get knifed.