While Michael's answer is true, it is also important to know that in recent times (In American dialects at least that I know of) "to do someone" means "to have sex with someone". I don't think it's applicable to your question directly, but it's something I think should be made known so that awkward situations can be avoided :)

It's a slang use of the verb "to do" which means to punish someone (in a way which is not defined) For example: "the police did me for speeding (driving too fast in my car)" You are more likely to "do" someone for something which is less serious than killing (!) but your example about the boss is still OK.

This is an interesting question and the answers are spot on. The phrase right now "I will do you for something" is inherently ambiguous and can have many meanings. I personally would articulate more clearly what I was trying to say. I will do you for something - HERE ARE THE POSSIBLE MEANINGS OF THIS PHRASE I will do something for you for something in return. I will have sex with you for something. I will kill you because of something (the something must be separately defined) I will sell you something. I will help you with something.

'I'll do you for doing something' means that I'll punish you for what you have done. Person A says to person B: 'I'll do you for killing the boss'. This means that person B has killed the boss, and that person A is very angry with him. Person A is threatening to punish person B.

"I will kill the boss for you" ^^