User blog comment:Gothamcity1992/Ben Killing/@comment-5808884-20140213134951/@comment-4050367-20140213211628

Malware wanted to destroy Galvan Prime, and bascilly kill all Galvans, Ben, and the Plumbers, espically AZMUTH when concerning Galvans. He wanted genocide on a galatic scale.

Also, Ben has been through a lot in these last four shows. Let's look at what has happened:

And I'm sure there's more I missed. Point is: Ben has been through more crap at his current age then any of us. I believe in a quote in Ultimate Alien, Gwen says something to Ben along the lines of "So that's you always makes joke, when you're nervous or scared". I don't remember which episode, but I'm sure she's said that somewhere. Ben has been through a lot, so that's why he's so happy and not mature, it helps him not be scared or nervous.
 * 1) He got a watch which changed his life forever
 * 2) Someone who he thought was his friend betrayed him.
 * 3) He learned his Grandpa had lied to him about his past
 * 4) He had questionable doubts about him being a hero
 * 5) He lost Gwen, for a while (also SotO)
 * 6) He lost his favorite alien to Malware, an alien that he felt he had such a connection with, that when Feedback died, it felt like a part of him died.
 * 7) His Grandpa went missing, and then later, he thought his Grandpa died.
 * 8) Aggreggor killed 5 aliens to absorb a baby alien to rule reality. Aliens Ben had befriended.
 * 9) His former friend turned enemy turned ally and friend became bad again, and now that his friend was so involved in Ben's life , and the danger he posed, if it came down to it he would have to be taken down.

And back to Malware. Malware is just too dangerous to be alive. Sure, he killed someone but would you rather have had Ben just let Malware kill everyone? No. At this point, I don't think there's anything else Ben could have done. It was a Superman/Zod moment in Ben 10. It was Malware or him.