My other rat died and my living rat doesnt like any of the rats I’ve introduced him

I brought home a rat after my living rat’s brother passed away, and there were times when they would get along, but they would mostly squabble, and ended up hurting each other a few times. We introduced him slowly and put him in the cage after they were getting along, and it seemed okay for the first few weeks. The other rat was not an aggressive rat, my resident rat was instigating it and would chase him and really attack him, and not in a playful way. Brought home another, same thing happened, and they had serious fights too. I’m not sure what I should do because he seems to be in a better mood now that I have returned them to the person I got them from, like is more energetic and playful and more interested in food.

I’ve never had this happen before. I’ve had rats for 14 years and I could usually get them to get along eventually but he got so upset with them sometimes he would start trembling. He was never like this with his brother, they really almost never fought at all and he was the submissive one. I know he would probably be happier finding a rat he liked but its hard to find someone who is willing to let you return the rat to them if the situation doesn’t work out.