Is the following a correct past perfect sentence?
I would have killed the snake if I had hit him hard with a stick.
It is a perfectly normal English construction and is an example of what foreign learners are sometimes taught as the Third Conditional. The speaker imagines something that didn't actually happen (in this case, he didn't hit the snake) and speculates what the consequence would have been if it had happened (he would have killed the snake).
Yes, it is. It's the third conditional, where you talk about an action in the past that didn't take place. Aside from that, you are missing the or a in your sentence.
I would have killed the snake if I had hit him hard with the or a stick.