Cat aggressive after nuetering

I just had my two male cats, Sully and Rex, nuetered 6 days ago. I got them from different places but they are only 4 days apart and I have had them since they are 10 weeks. they are now 15 months. They have always gotten along very well and takes naps and play together. After their surgery they had to wear the cones because they kept trying to lick the area. When they came home they were sore for a 2 days and then  Rex was back to his normal self, Sully still kinda slept a lot an dnot his normal amount of energy but then 2 days later (yesterday) fine walking around and being active but still walking a little funny like it still bothers him. They didn't hang around each other as much but still would come up to each other and greet one another. Then last nightout of nowhere Sully started attacking Rex VERY aggressively, so bad that he got Rex near his incision and made him bleed. I immediately seperated them into two different bathrooms. Sully was still sweet and friendly to me. Sully has never been aggressive towards people or Rex, so I was shocked that he was attacking him with such intensity. This morning I took off Sully's cone because I though maybe the cone is giving him an attitude ( he looks super unhappy with it around his kneck) he was excited to have it off. I let them see each other again and 45 seconds later Sully attacked Rex again biting him hard in the back pulling out a huge tuft of fur. Is it possible for cats to completely change their feelings about another cat they've lived with for so long after being nuetered? Is there anything I can try to lessen Sully's aggression?