Kenshin Himura...
"Talk about a strange day. First, I end up in a completely different universe from my own by this person wearing a bat costume calling himself "Batman", then I am told that this Earth was being invaded by this technologically advanced being, apparently calling himself an "alien" lifeform called "Brainiac" and that if I was to get back home, I would have to help stop Brainiac.
Speaking of Brainiac, his drone army were quite numerous so due to them being metallic in nature I didn't have to hold back my slashes. It sort of reminded me of those days when I was the Hitokiri Battousai during the Bakumatsu war.
As much as I didn't want to be reminded of those days.
With each drones I sliced up, I could feel the darkness within me just begging to be let out. I knew what that part of me was.
The part that I have spent my whole life trying to repress.
The beast within that would just kill everyone on sight.
My Battousai side.
But luckily, it didn't get the chance to come out as I managed to beat it back and slash the last drone down.
With the drones dismantled and the people defended, all that was left was stop Brainiac himself.
And to be honest, I believed that he was quite possibly one of my toughest adversaries yet.
I believe that even Shishio Makoto wouldn't compare to him.
However, with my disciplined mind and highly observational skills, I managed to restrain myself in order to focus on stopping Brainiac, which was extremely difficult to accomplish considering the "alien" being stronger and smarter than me as well as having the innards of his ship backing him up.
Waiting for the right opportunity, I managed to cut through the alien's armor and slice his tentacles to pieces before putting the rest of my remaining strength into a single strike as I managed to knock him out.
When the invasion now behind me, my killer side and rational side began to argue with each other. My Battousai side wanted to kill Brainiac for the sake of the multiple planets he has destroyed while my rational side argued that if I killed him, it would just make no better than him and would DEFINITELY make me no better than the deceased "Joker" or the rage and grief filled "Superman".
So I spared him as Batman and his team try to use the ship's technology to restore the cities that Brainiac had stolen.
With this world now coming back to a state of normalcy, Batman sent me back to my time.
As I went through the timestream, I mediated to calm my mind.
I will conquer my inner demons no matter what.
After all, I vowed to never take another life.
And I was not going to stop now.