Technically mass genocide is very mature stuff. So is emotionally abusing your son so he does what you want him to do unintentionally and trying to enslave a region by ridding it of Trainers, or wiping reality clean in your own image with no spirit or any of the things that make us human or alive.
I think your problem is that it wasn't handled well. Lysandre's goal of wiping out all life and making his own team immortal is more of a shocking swerve than anything else, as we're given little motivation for him wanting to do it, little foreshadowing that that's what Team Flare's endgoal was, and at no point was it ever handled in a serious manner. You didn't feel like the world was at stake, or at least not as much as you did playing Platinum or B/W, and Lysandre was never developed as a villain or even as a character so you never really saw him as a threat until AZ shows up and tells you exactly what he's going to do. It was poorly thought out, and the fact that nobody but your friends tries to help only makes that more apparent. If Lysandre was presented as more of a mad man and he had more screentime it probably would have been more impactful.
