I finished tribunal a while ago and chose the side of heleseth intead of the the temple and I never found out if heleseth really did poison the king and poisoned his nephew, what happened?

It is never explicitly stated but when you support the King, his captain almost admits that he did it. Supporting the temple doesn't give you any new info. You do the same quests for Almalexia until you have to get the sword piece from Karrod and then deal with the king.

Given Helseth's history (read A Game At Dinner. A copy can be found in the Vivec Mage's Guild) and all the proof that he did, in fact, kill Athyn Lethan and his nephew, it lead me to believe that he did murder them.

Remember, this is the man who uses his own mother in a plot to test the loyalty of his retainers. The lives of others seemingly mean very little to him.

I have to agree with malchik and cmac on this. It does "seem" that he did kill them, and considering the account I read in "A Game at Dinner" about the "test of loyalty" that he gave his followers, he has used poison in other acts of killing. (Tell me that bowl of soup I just ate wasnt leftovers from Helseths table...)

better than the temple, maybe Heleseth isnt very nice but I would side with the imperials no matter what