Saitama!
"I'll admit, I have faced a ton of different foes and all it took to beat them was with one punch. So that alien planet collector was no different when transported me to his universe and tried to add me to his collection.
My answer was quite obvious when he attacked me.
When what's his name was went down, someone even worse tried to use the ship's rubble and whatever tech that survived the crash to start his reign of terror on this Earth.
I wasn't going to allow that to happen.
But despite what I did to that alien, I decided to restraint my power in order to not kill him as one of the heroes of this world, a guy dressed like a bat, wanted to imprison him.
I understood why considering the fact that he told me that the two of them were close friends.
So once the caped psychotic wonder fell, and that bat guy imprisoned him in another dimension, I asked the bat guy if could help get me back to my universe.
He chose to help me, but warned me that should I try to invade his universe like that smart alien did, he'll take me down by any means necessary.
Considering how powerful I am, I don't blame him for being paranoid.