Rat over a year old suddenly gets mean...?

ok so I have 2 rats that are brothers, they're about a year old give or take a month or so. They've lived together their entire lives. Lately one of the boy's, Slip, has been acting weird. 2 weeks ago we though his brother Rufus cut his foot on something in the cage so we separated them till he healed (3 days). then we noticed after we placed the 2 back together Slip was getting into a defensive mood and would stand on his hind legs trying to ward off Rufus. we didn't think much of it because they play a bunch and never have we heard them fight or even squeak at each other. I mean they dont even do the dominate power grooming thing with each other. they'll usually will stare at each other for like 30 seconds then go p out in a corner together and all is well. Well last few days we've been seeing a little more action between the brothers nothing harmful just fighting over who gets what box for the night to sleep in or one would steal they others Lima bean. but what threw me off is last night we took out ALL of our boys to have free time on the bed. that's 6 adults, which we NEVER have issues with free time between any of them. they all know each other quite well. but Slip started getting really pissed when everyone got around him. he'd push them away or swing at them. if we tried to get near him he's turn around and act like he was going to attack us. and at some point he took 2 big chunks out of his brother Rufus, which was weird because we didn't hear any fighting or squealing. just a bunch of blood slathered all over our bed. so now we're sure Slip injured Rufus' foot 2 weeks ago and we know he did the damage to Rufus' back and front leg. so we've separated them so Rufus can heal, and Slip can't harm anyone else. My question(s) is, 1. WHY would a cage/litter mate of a year old suddenly become so mean to everyone and everything that comes into his path? and 2. Rufus' front leg that got injured last night swelled up to a pretty good size, we cleaned the wounds dressed them with neosporin and let him be, he's not favoring it, he's walking normal but it 5 times bigger then it used to be. I know i know.. go to the vet, but my vet said unless the wound hasn't clotted, which it has, or he stops walking on it that she has no need to really see him. So should i just keep alternating warm compresses and neosporin with warm compresses and Honey to help heal him up or is there something better for a home remedy that I'm not thinking of?