How to deal with a bully?

Question
Hello!

I've been going crazy past month or so trying to figure out what to do with one of my bucks. He has become beyond ridiculously aggressive toward his cage mate to the extreme that he tore him open on the side. And I've had him on isolation from that day on, every time he see's his cage mate he puffs up and just tries to maul him. I dont know what to do at this point I dont want to castrate him just yet any tips on how to deal with him? By the way the cage they shared is predominately big enough for both ratties. I feel it's my fault I saw them fighting previous to this attack and never took much notice just thought it was normal rat wrestling.

Thank you so much!

Answer
Unfortunately, since your cage is large enough, you don't really have a choice other than to have him neutered if you want the boys to be able to live together peacefully again.  

You didn't mention his age but I'll assume that the boys were rather young when they were getting along and you started to see the aggression when they reached adulthood.  This is fairly common with male rats and neutering will most likely resolve this problem

Please let me know if I can help you with anything else and good luck with your boys.