Question

[kindaichi]

WHich one is correct when you ask someone a question?

Did you killed her?
Did you kill her?
Do you kill her?

And for this question,

Never do that again,understood?
Never do that again,understand?Which is correct?

[Mister Micawber]

.
These are correct:

Did you kill her?
Never do that again,understood?
Never do that again,understand?
.

[Mariner]

Did you killed her?
Wrong

Did you kill her?
Correct

Do you kill her?
Grammatical, but very strange. It would imply a habitual behavior (i.e. that you kill someone repeatedly, for habit)
EDIT: Come to think about it, still we would say "This thing is killing me"

Never do that again,understood?
Never do that again,understand?

Both are correct, but I dare say both are a bit informal. In a more grammatically correct (or formal, if you prefer) way, they would be phrased as:
"Never do that again, is that/have you understood?"
"Never do that again, do you understand?"