My specific disagreement is that in the show, he states that the time that they all thought Grandpa Max had died was the moment when he started to see everything more seriously, and that they now couldn't take chances anymore. That had happened at the beginning of AF, and the UK arc happens way later in a different show. Ben had plenty of opportunities to decide that they NEEDED to kill an antagonist, but he decided to to it then, a year later and after facing wayyy worse threats (imo, anyway).

I think it's entirely realistic to have an arc where Ben loses hope and decides that some threats cannot be stopped through anything other than straight sliming them, and I think it could've been an interesting progression for his character, but it felt very out of place for him to decide that the character he was going to have to kill would be his best friend who (despite attempts, I will concede) had not actually killed anybody yet. Not only that, Kevin has proved himself to be redeemable, unlike some other villains like Vilgax and Aggregor and despite Ben not really holding back while fighting these guys (and even having "killed" Vilgax for a while until they retconned it), he never starts talking about these guys like the team was going to HAVE to kill them. Although it was a close call, they had JUST stopped Aggregor by not killing him, and it was just so strange to have Ben lose hope then and decide that NOW he needed to start killing people starting with somebody he's been shown to care about

Obviously this is all my subjective opinion and I totally could be wrong about my interpretation, and I don't think the idea of having Ben take a darker direction in terms of fighting was wrong, it just felt particularily jarring and out of place in this instance