I don't think Nick Lacarno was unredeemable. He was reckless and got a teammate killed, and then he covered it up. An act of recklessness followed by an act of fear. How many real people do you know who, if they really screwed up, would come forward and admit it? If he served his time in prison, then he learned from his mistake and worked earnestly to make up for it, he could absolutely be redeemed.
I actually think it wold have been more interesting if Tom Paris had been more like Nick Lacarno.
Tom always felt like a nice guy that made a mistake. They tried to paint him as the anti-hero in Caretaker, but after a few episodes he became just a nice as everyone else in the cast.
Someone like Nick would have had a harder time on Voyager, maybe with more of the crew hating him, which would have been more entertaining. He could have been more of a bad guy trying to be a good guy. Also having Janeway mistrust him much more would have been interesting.