It is perplexing that it took 4 issues to get to this point.
I get that he has to reintroduce things and establish the world all over again but I really think this would have benefited from being a prose novella with illustrations ala What Savage Beast leading into the sequel mini which has already been announced.
As we already know, a merged Hulk is the weakest Hulk due to Banner's limiting control. Or as Herc best put it, Banner knows he'll get beat, ergo he's going to get his ass beat.
But as Rick said (paraphrasing) add in a crazy Banner, and who knows what kind of Hulk you'll get. Or in this case, a very pissed off, vengeful Hulk. A Hulk who consciously choose to kill two people in a calculating manner. This isn't the Hulk we've seen 4 issues now.
Banner's pride got the best of him and this is what happened. And maybe this is reading too deep, but the beating he took may have reminded him of his dad (the last real ass kicking he got -- not that his dad is flipping him around by the big toe, although an adult Brian attacking a child Bruce could feel that way)
Again, something that would have been explained with prose better, or if they inserted flashbacks during that sequence. But that would interrupt the Avengers joke.