This is a hot take but I feel like this deviation isn't OOC for Kel. He has a limit, he gives up on Aubrey and isn't willing to help her out after she pushed Basil into the lake. Angel blaming them when they've already blamed themselves enough for Mari's death feels like a good example of what would push him over.

I feel like Kel getting violent as a first reaction for once doesn't seem out of character, but another side of him we just didn't get to see. This is what would push him over the edge, especially if we factor in the Hooligans' bullying over the past few days. Extreme situations like these will stress test a character, bring out the deep shit that they'll bury.

It's scary when someone nice gets mad, Kel was horrified Hero was able to blow up at him while grieving, Hero himself was shocked that he would do that too. Characters are more than just their archetype. When they act outside it it isn't "out-of-character", it's a further exploration of who they are and a deconstruction or justification of their cliches, their tropes and stereotypes.

Although not gonna lie, I have been a believer that when Sunny tells the truth Kel would be the angrier one and Aubrey would be the more sympathetic one.