Complex sentence: For a while, until his formal introduction, Mikoto observed the Abarangers from the side while mastering the DinoMinder and coming to the conclusion that it be fun to fight against the heroes, thus naming himself "AbareKiller".
Split into simpler sentences: For a while, until his formal introduction, Mikoto observed the Abarangers from the side while mastering the DinoMinder. He came to the conclusion that it would be fun to fight against the heroes, thus he named himself "AbareKiller".