I also think that Alex himself isn't sure why he attacked DeLuca. I'm sure that a part of him thought DeLuca might be taking advantage of a drunk Jo, but much of it was just anger and jealousy, but he wants to believe that it is something other than that. He even admits as much to Meredith. He says, "I saw him on top of her and I just lost it." That could be taken more than one way. (And I say this as an Alex superfan).