In Act 3, Scene 3, Hamlet does not have the ability to act when he sees Claudius, praying, when he is at his most vulnerable because Hamlet's psyche and conscious motion towards certainty takes over. Overall, as Old Hamlet's 'sole son [he will] do this same villain send To heaven.'
Hamlet recognised now he is praying', which is evidence of his guilt, due the fratricide upon his brother and Hamlet's father, Old Hamlet. 'Why, this is hire, and salary, not revenge.' - Hamlet would have liberated, Claudius of his guilt, it would be rewarding for him. Hamlet's logistical mindset is what fuels his tragic flaw.