Hmm. A few things going through my head.
He went through the courts, it was determined that he didn't intend to kill someone. He served 4 years in line with that, he wasn't charged of sentenced because he murdered someone, he was charged and sentenced for manslaughter. Theres a whole world of difference.
Have you never been in a fight? I know as a teen I had fights with others, and I can only say how horrific the thought is that one of those stupid fights could have ended up with someone dying is. That was never my or their intention I shouldn't think. In part I'd feel quite sad that the situation turned into that. Imagine a stupid fight, maybe even just one punch ending someone's life. That's some guilt to be carrying around for the rest of your life.
I'll admit that I wouldn't want to be your new neighbours best friend. Fuck that. I'd keep my distance, but you've allowed a neighbour gossiping, and their spin on the situation quite a lot of head space.
I would likely avoid them, I'd be unsure whether their temper was that bad, or was he actually made calmer by the fact that he's seen the very worst that can happen when you lose your temper?
Ultimately its all very sad, but the likelihood is that he wasn't the only one acting like a twat the night that someone's life ended. You don't know what happened, you don't know him, you know the narrative that was given by the papers and your nosey gossipy neighbour.
I'd keep my distance, but honestly, I'd not take one fight as a reason to think he didn't appreciate life, and write him off. If he was this terrible terrible person, he would've been in prison for longer, and wouldn't be in his exes house around their kids after being in prison. If she wanted to keep him away, she would have had plenty of help to keep him away after being in prison for that