commandToClient(%cl,'ServerMessage','MsgYourDeath',"");
this is how you send the death message to the client, to do it client sided you could probably package servermessage and check if its "msgyourdeath'
damage is a bit trickier, the only way i could think of doing it would be to check for damage flash, which could be affected by other mods
the only way you can detect if you kill a player is if it's sent in a chat message, otherwise I don't think there is another way to detect killing of other things