Tony wore his emotions on his sleeves, I figure not killing that witness was where his life in the drug game went wrong. His life would have went in totally different tone had he done what he was expected to do.
He might've been weighted down morally and still went on a coke binge, but he wouldn't have had a death squad after him going through with the mission. It's likely he would've been feeling like shit for killing the children along with the witness. Doing so would have violated his own code as a man, guilt would've been kicking the shit out of him. Probably would of been too burdened with guilt to kill Manni. Probably wouldn't have had the conviction to kill anyone again so easily and so soon.
His business would still be flourishing, his relationship with Sosa would be intact, others in his world would of had even higher respect for him. But he'd be wrestling with himself for crossing that one line he thought he'd never cross. I'd say Don Micheal Corleone is a close example of the man Tony could of been had he blew up that car.