I know there's always been some violence in Pokemon, that some of them are predatory and stuff. I don't even mind some of the darker entries for some Pokemon like Palossand for example. But it's still supposed to be a lighthearted franchise for kids at heart, so there is certainly such a thing as too much darkness. For own thing, it looks like most of the Alola entries have these dark undertones to them, whereas they'd only be one of the occasional ones in the previous games.
I mentioned that my problem was specifically with the Mega Evolution entries, because they generally say that either the Pokemon is in a lot of physical pain when it does so, or it turns into a heartless monster. This is in stark, stark contrast to how it was portrayed initially as a representation of the friendship between the Pokemon and its trainer. I feel like it makes Mega Evolution less appealing as a concept because it seems to involve a lot of suffering all of a sudden, like you are almost a bad person for using it. It may have been hinted at in the anime (and only once, my impression is that all other Megas were fine there?), but this is way overboard.
Similarly, I think it's kind of crazy to go 'yeah, the previous games exist in another dimension, but the characters you know have been kidnapped and their memories wiped, and also the world might have been wiped out by these monsters'. I don't know who this plot is trying to appeal to - it won't mean anything to younger players, but to older players I can't see it being much of a consolation to say the universe they know existed but is now gone rather than just being retconned out. That, and this is my personal taste, but I just feel like all these multiverse splits between the main games feel a bit much. I don't feel like it is necessary over than to make remakes and the originals exist at the same time for some reason, and it has shades of almost a conspiracy theory to me which doesn't seem to quite belong again with the simplistic, kid-friendly plots in Pokemon.
Seriously, I feel like people are allowed to complain if there's something they don't like. You can't just say I 'lack understanding' for not liking how a concept has gone from relatively mild to horrifying all of a sudden, and you can't just tell me to go away. If you like the darkness, sure, that's fine, we can agree to disagree. But a tonal shift like that is going to turn off some people, and I just happen to be one of them.