In "Hamlet", why doesn't Hamlet kill Claudius when he discovers him in an attitude of prayer?
Hamlet wants for Claudius to go to hell, so he does not kill him while he's praying.
At the time, it was believed that if one prayed and repented for their sins, they would go to heaven. The catch is that when Claudius killed Hamlet Sr., He was unable to atone for his sins and wound up in hell.
Knowing this, Hamlet did not want to kill Claudius while he was praying, because he did not think it would be real vengeance if Claudius ended up in heaven while his father still languished in hell.
Ironically, Claudius wasn't praying at all and it would've been an ideal time to kill him.