Eh, Ben’s change in his Behavior are somewhat justified. For one, AF takes place five years later, and it would reflect really badly on Ben really if after the entire OS + five years he still hadn’t learned a goddamn thing. Another is the fact that they wanted the Highbreeds to be taken seriously.
One of the big things they seemed to be going for with the Highbreed Arc was that it was supposed to be a real threat.
Subversive conspiracies, Kidnappings and conversion of random people into twisted mutants, being on you’re own never knowing who you can trust, the absolute badass that is Max being pushed on the run, these weren’t meant to be your regular old fun paranormal/extra-terestrial encounters from the OS. The Highbreed were supposed to be a big fuckin deal, like OS Vilgax, and fighting them wasn’t suppose to be a matter of playing big damn hero, but a Grim, Dark, bleak, shadow war, like Animorphs.
Now, say what you want about the intended concept itself, and how well the executed it other areas, but it is undeniable that the key to have something be taken seriously by the viewers, is to have it be taken seriously in universe. And having Ben, smarmy, grandstanding, gloryhound, Ben Tennyson, A character known for being a bit of a smarmy cunt with a tendency to let his ego swell, shut his mouth and treat these guys like they mean business. That did better than any powerlevel wankery at letting you know that shit had gotten real.
Also Kevin’s character would have been fine if they had had at least one segment, where someone went Hey, Kevin! You’re not a mass-murdering psychopath anymore! That’s New!” Instead of trying to pretend Anti-Hero Kevin had always been a thing.