Hamlet Act II

In Act II, when Hamlet is trying to kill Claudius I dont understand why Claudius doesnt realize that Hamlet is behind him. For some time I thought it was because he was praying but I am not sure. Even though Claudius basically took Hamlet's fathers place in Gertrudes life doesnt mean that Hamlet has to hate him so much because at the end of the day it is still Hamlet's uncle and i think that he should be respected as that. Also, I feel that Gertrude doesnt even care about how Hamlet feels about the two of them. Not that Hamlet should have any say because it isnt his life but i do think that the right thing to do is to take his feelings into consideration. After all, when Hamlet's father died Hamlet has a very big void in his life now.