Even if the villain that was killed was a piece of shit like Black Mantis, when many such villains like him die in a short amount of time, there would be repercussions. So heroes mostly did lip service and took down low-time villains once in a while.
But it seems the times have changed, and the heroes want a piece of them again. This suspicion turned out to be true. Fire Rose and the Justice League were warned, but Fire Rose responded by killing another villain a week later. It looked like war was really on the horizon.
Kelvin didn't know all of these. He was busy with the preparations for his advancement game. What he needed to prepare for was a lot, so he was very busy.
It was as if he was preparing for a difficult exam. There were to be a hundred competitors in total. So there would be 99 enemies for him other than the beasts in the game.