Apart from wanting to confront Pan, Rumple considers himself a man who sticks to certain values and killing someone while said person is asleep is the lowest (sorry Regina 🙁 ).
He knows that he has done a lot of horrible things and probably didn’t want to add another thing to that list. And as RG said he wanted to talk to Pan, though I don’t think he wanted to gloat and show him he had won, but maybe he hoped for Pan to show regret, similar to Cora who at last told Regina that she would have been enough. Rumple might have known that his father was a bad guy (I can’t say the word I want to because kids might read this 😛 ) but nonetheless hoped for some kind of reconciliation.
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