There was no point in staying on Earth.
If he was going to disappear from his sister's life; if he wouldn't be able to watch any new episodes of Italian Mafia Reborn since according to Hera, most of the people that worked on it are now dead— there was no point in him staying on Earth.
And so, when Paige suggested that he try and rule the world, a little tiny excitement was born inside Riley. Finally, he thought… finally he had something to do on this planet. Killing and torturing people was nice, but there was also no point in that since most of the humans were already down and beaten.
Most of them are probably already numb to death and pain right now because of all the tragedies that have been happening around them.
