Did I missed something or only way to finish Blackwell Legacy is by destroying Deacon's flask/sins? I don't like this, I haven't even a bit of sympathy for this pathetic fu*ker and wanna watch him burn in hell but I can't.

This little game was really nice until ending that made me upset as hell. Did anyone had the same feeling?

Nope, you don't have to save him. There's a time limit for destroying the flask; if it's exceeded, the bad ending is triggered.