Hamlet doesn't act decisively because Claudius is praying.
Explanation:
When you pray, your sins are forgiven, your soul is almost pure. Hamlet believes and claims that if someone is killed after having been forgiven for their sins, they would go straight to heaven. He doesn't want that for Claudius, he wants to actually make sure he goes straight to hell.
However, this scene is a paradox since in his prayer Claudius does not express enough regret for killing King Hamlet, and thus his sin isn't even close to be forgiven. Unfortunately, Hamlet does not know this and he misses the oportunity of taking Claudius life.
