There are two things that I'm going to address here with this season finale in Ultimate Alien. One, Ben's decision to kill Kevin and two, what I think is a missed opportunity with Ultimate Kevin. Now it has been a while since I've seen Ultimate Alien so if I get anything wrong, feel free to correct me on it.
In regards to Ben wanting to kill Kevin, I recall seeing many in discussions argue that it was an out of character moment. That it wasn't something that Ben would've actually done. I don't think that is what makes his decision to kill Kevin the problem here. I think it's more on the matter that it wasn't handled well. Maybe even came out of nowhere and had little to no build up or something.
The idea of Ben being pushed to kill someone is actually a solid direction to take the character in and this isn't the first time this actually happened in the franchise either. The Ben 10,000 episode in the Original Series established that there would be a point where Ben would kill Vilgax and do so in a rather brutal and gory fashion. In fact, the Little Moments fan fiction series very much indeed has Ben kill Vilgax in the very fashion that the Ben 10,000 episode implied and I say that it was handled quite well. This is an event that would impact Ben's transition into Ben 10,000. Heck, the Ken 10 episode showed Ben nearly beating Kevin 11,000 to death because Kevin harmed Ben's son, Ken. It wouldn't be out of character for Ben to be pushed into killing someone, but instead, is an important character moment for him. I just don't think that the Absolute Power season finale in Ultimate Alien handled that well.
There is one thing that I do think was certainly out of character for Ben to do and that is attack Gwen. Given everything that those two have been through together, how much their relationship has improved, and how much they care for each other, I don't see Ben EVER doing that. I had that same issue with the 3D CGI Ben 10 short that was meant to promote Destroy All Aliens as well. No matter how much they fought and argued with each other, I don't see them going so far as to use the Omnitrix and magic to literally fight each other. That was one of the dumbest and nonsensical moments in Ben 10 and their fight scene with each other in Absolute Power was also dumb and nonsensical.
Then there's Ultimate Kevin. I've stated plenty of times before on how I prefer for Kevin to be a villain instead of a hero and explained my reasonings as to why. I think it just seems more fitting for Kevin to be a villain and to fulfill the role of being Ben's evil reflection. I think Ultimate Kevin is a missed opportunity to put Kevin back into that very role. Not just that, but also act as a good and fitting moment that would significantly impact Ben's transition into Ben 10,000.
I stated this in a Ben 10 Discord server that I'm in so I'm going to share it here as well. I think how I would've handled the Absolute Power finale is not have Ben decide right away that Kevin needed to be killed. Instead, have Ben contemplate how he should handle Kevin. To make a tough decision of whether if there still is a way to help Kevin or if Ben really is forced to have to kill him in order to stop him, an option that Ben doesn't want to pursue. After all, Ben is a hero. He wants to help people. Ben and Gwen work together to hatch up a plan to help Kevin. They don't fight each other. They work together.
I think almost everything else from that finale can pretty much stay. Darkstar and Cooper are recruited and preparations are being made. Kevin absorbs some of Gwen's mana, thus painting a target on her, which makes Ben concerned. Kevin just hurt Gwen and the plan relies on Gwen intentionally putting herself in danger and being bait. Maybe the plan is starting to get too risky now, thus Ben contemplates his options of killing or saving Kevin again for the reason of wanting to protect Gwen. Ben cares about Gwen and doesn't want for Gwen to get hurt or worse.
The plan is enacted and then things go wrong. The plan falls apart (maybe at least some of the energy and DNA that Kevin absorbed is taken out of him to be given back to his victims). Maybe Darkstar betrays the group and sabotages the machine that was supposed to turn Kevin back to normal. Maybe Kevin has too much energy and DNA in him and it causes the machine to overload and malfunction. Either way, Kevin doesn't return back to normal. Kevin is still in his mutated and insane state and he goes after Gwen. Gwen's life is in danger, pushing Ben to his limit and he nearly kills Kevin to protect her. The plan is a failure and it seems that there's pretty much nothing that can be done now to help Kevin. Kevin is too far gone and all that can be done at this point is to toss him into the Null Void. Ben and Gwen are deeply impacted by this event, thus contributing to Ben's transition into Ben 10,000.
I think this is pretty much how I would've handled Absolute Power differently here. I think this would make Ben's decision to kill Kevin seem more fitting and better handled, Kevin would be a villain again, and this would be a character defining moment that would impact Ben's transition into Ben 10,000. If anyone has any thoughts of their own on this topic, feel free to leave a comment.