What should I do about my solo aggressive rat?

[Carly]

I originally got my rat Howard as a cage mate for my other rat who had been living alone for some time. I did everything I read about online, quarantined him, put vanilla on the base of their tails, and introduced them in a neutral area. But the introduction never went well. It always ended in Howard attacking the other rat. The final time I tried, he drew blood, and I didn't try again. After the first rat I had passed away, I ended up with another. I also tried introductions with this rat and Howard, but it ended the same again, but this time a worse wound. I felt so guilty. He's a really sweet rat to me, and has never attempted to hurt me until a few months ago. I was holding the other rat for a while before I got Howard out, and really had the scent of the other rat on my hands and arms. I noticed his fur started getting all tufted up, and he was walking around kind of funny, when a few seconds later he attacked me, and bit me. So, I have been washing my hands before holding him, and letting him run around in places the other rats don't go (so he doesn't get aggressive because of the smell). I mean, he seems like a happy rat, but I keep reading online that it's not good to have them alone. I just don't know what to do about that. He is not fixed, and I've thought about having him fixed, but he is about a year and a half old now. I read that the risk factor for complications goes up with age? Some advice would be nice.