Guppy pecking

First time posting here, sorry if it's in the wrong category, though I hope it's the correct one.

So I got these new guppies after stop keeping a tank for a year or so. One died giving birth so I'm left with a bunch of babies and four guppies left. Two males and two females. And then a female, but I could find absolutely no reason to why she died, except for having some wounds and a slightly ragged fin, she looked fine, I was kinda worried that it was fin rot. And the a few days later a male died, but I noticed the other male kept bugging it, nipping it. I looked up online, but didn't find anywhere that said that could cause death. But as a precaution I separated the remaining two guppies, and they both have lived fine for over a week now, meaning there's no disease in the tank.

After about a week I put the male one back in with the female and he immediately started nipping her, so I separated them again(the female is slightly pregnant, so he can't be trying to inseminate her) a few days later I tried putting them together again. This time he was even more aggressive, biting, not just nipping the female, to the point that he had a bite on her while she was trying to dislodge him. I am almost certain the other two were killed by him too. I've never seen a fish that aggressive before.

What should i do? Should I just killed him? I can't possibly separate them forever. (That's a last resort, he's really quite beautiful so I'd like to have a strain with him. But I'll do what's necessary to keep the remaining female and the babies when they move in safe)

I just went to check on the female. There is now a very small rip in her dorsal fin where he bit that I'm quite positive did not exist before. He was only with her for less than a minute and did visible damage to her, I can't imagine what he would do if I left them together.

It's a 7 gallon tank. Ammonium level is normal. Aside from the male I noticed no sign of the fish breathing heavily indicating ammonium poisoning, and there is no visible change to their physiology of bacteria, bloating etc. Only shredded fins and ruffled scale. I thought it might have been fin rot, but now I think it's the male that caused the fins to be that way. He was aggressive the first day I bought him back when there were four females, but I figured he'd settle down after a few days. He did not.....