Aizen is precautious and perhaps he foresaw Tosen's betrayal after Ichimaru's. That was probably why Tosen's death happened as he died after getting beat up by komamura and hisagi so badly. ... However, he should've been able to sense his reiatsu fall and thusly killed him.

Why did Aizen want to kill the soul king?

Aizen obviously being Aizen he found out what that sin was and what the Soul King was. That's why he called the Soul King "...that thing". He knew that the Soul King didn't govern Soul Society. ... So Aizen's goal was basically to take the Soul King power for himself.

What happens to Tousen?

In ep 291 of the anime, when Tousen is dying, he suddenly seems to just explode in a burst of blood; the rock he was lying on is suddenly covered in blood but his body is totally gone.