What happened to you? <--correct- because whatever it was, it was in the past
What happen to you? <--incorrect. In the present it would be 'happens' and it would need more context because it would be a repeated action- 'What happens to you when you get home late?'
What killed you? <--correct? why? Correct, but why are you asking a dead person a question? They can't reply.
What kill you? <--correct? why? Incorrect
What harmed you? <--correct? why?
What harm you? <--correct? why? Incorrect
These examples are basically the same- the past makes sense in all of them. The present would require more context and would always have an 's' on the end.
What brings you here? <--correct? why?
What bought you here? <--correct? why?
Both are cporrect- it depends on the timing of the question. I'd use the present when the person is just arriving and the past later on.;-)