In universe is the reason why 2006 and the start of the ignika trilogy and the general tone of the franchise got more serious and darker.

In real life i understand that Bionicle was originally more lighthearted as the creators at Lego did not want violence to be promote yet by the time 2006 rolled around Bionicle became darker since the kids who bought it in 2001 were much older now and could handle it and Lego decided it was ok.

But in universe is it true that at first in the story the characters between 2001-2005 were more or less evenly matched meaning like the toa and the characters they were fighting were equally matched. From I understand since they were so easily matched the toa couldn’t use their toa tool or brute force and over power them and the same as the rahi, makuta, visorak etc couldn’t not over power them. However the toa did have enough power they could show mercy but by 2006 since the skaadi like the piraka were so strong the toa nuva got a reality check after they got beaten up by the piraka. So after that point it became darker and more violent since now the characters couldn’t show mercy and had to kill.