Sam should have just killed Lester

In the context of the movie he should have. It's not like he or Nicky were afraid to play rough with anybody else. The movie was plenty long as it was so Lester could have been cut out earlier and not lost much at all. Ginger had plenty of other low life friends if Scorcese wanted Ginger under somebody's thumb. The bosses would have only been too happy to make Ace happy by getting rid of an annoyance. I am surprised Lester was willing to continue on after he got the scent of organized crime that had to be rolling off of Ginger. It would have only taken one beating for just about anybody else to run off.

You kidding me? It's the beating he gave Lester that started Ginger hating his guts in the first place. If he had been smarter and handled this differently, he wouldn't have much problems with her.

Then again, Ace is kinda stupid and arrogant. He could've given Joe Bob Briggs a simple as simple job like a shoe-shiner but he couldn't even do that. It's the tiniest mistakes that sent himself down a fall.

Man is the cruelest animal. ― Friedrich Nietzsche

Movie Ace should have killed Lester in an "accident" so Ginger couldn't blame him.

The real life Ace couldn't kill Lester because he was Ginger's baby daddy.

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Sam needed his clean image and it could be hard to maintain that if Sam had someone murdered.

Nicky was sent to protect Sam and his image by taking care of any dirty business that needed to be done.