Culpable
 

How can we say ¿ Quién es el culpable de la situación?

Who's guilty of the situation?
Who's going to take the blame for ?
Who can we put the blame on?

Your last two questions are good (add a prepositional object to the first of the two).

A good way to use 'guilty' is:
Who's guilty in this situation?

Also this is very commonly used: Who is the guilty party?

All of those options sound overly literal to me. If you're looking for a more dynamic translation, I'd say Who is responsible for the situation?

thanks

Who is responsible for this situation, is neutral. It can refer to a good situation or a bad situation. Who is the guilty party refers to a bad situation almost exclusively; the exception being if the person uses an extremely ironic tone.