Sinister, I agree with you in most aspects.
When you look at him, from the moment he kills Sollozzo and McCluskey, everything seems to be a burden for him.
I think we see the burden earlier. It begins when his father gets shot. He realizes how bad things are being handled by his brothers. He sits frustrated on the bench outside the house. Also when they discuss whether to agree on Sollozzo's proposal and who is going to be killed, he sees that the ship has lost its captain and the navigators are no good. Look at the way he hangs up the phone after Clemenza says: "Luca Brasi sleeps with the fishes."
