Well, let me tell you why!

Because I was planning a mercy spree!

It could also be considered yet another parallel to the original fallen child – the little kid who climbed Mt. Ebott because they despised humanity with all their heart.

For all the horrible things he planned and did, Chara was still a kid at the end of the day – that hate had to come from somewhere, especially in a game that encourages looking below the surface as opposed to killing monsters for appearing monstrous.