They can play with his emotions right up to the finale and I won't care. If in the end they don't let him find his redemption on the island, what is the point of the whole show?

Not every character will find redemption. That doesn't strike me as plot abuse - in fact, it stands to reason that some will pass and some will fail.

Mulling it over a bit, I see what you mean though. Sawyer's turn back to the dark wasn't terribly convincing. Maybe Jack should have retained more feeling toward Juliet in the buildup to her death, giving Sawyer more reasons to resent Jack. Maybe his action should have led directly to her death instead of he getting dragged down a hole by a chain.