Yeah, Toriel I definitely think was just mocking Asgore's cowardice and violence when they could have possibly waited the situation out. How old do we think they are, anyways? There is obviously the Asgore looking guy in the flashback, but I think this must be an ancestor, because they establish the soul transfer/inheritance system with the turtle guy's endgame dialogue, and I don't see why this'd be brought up without there at least being some generations of his family before him while the monsters were trapped.
I also get the impression that the turtle wouldn't have known this without at least being aware of some generations of the royal family prior to Asgore's, and if Asgore had lasted since before the war, why would someone who had lived through "most" but not all of history know about it? I guess it could be common knowledge but it seems like something that wouldn't be talked about a lot and that is sort of forgotten by most monsters since no one tells you about it before. Additionally, I just don't think Asgore could have been around during the war, since he doesn't seem to hold a grudge against humanity for it, but rather a sorrow from the death of his son & Chara.
Another thing to consider is how many other "boss monsters" are about, since I get the impression Asgore was the royalty and Toriel was wed into the position, since she refuses to be called "Dreemurr" anymore.
I choose to interpret Chara was a kid who for whatever reason had an abusive life and may have tried to kill themselves by jumping down the whole, but found some happiness when they were adopted into the royal family. For some reason however, the darkness remained, and they wanted freedom despite being somewhat happy for a time underground. So they executed their xanatos gambit suicide strategy to escape with Asriel (I think they wanted to live inside Asriel, not control him for evil reasons), but Asriel stopped Chara from destroying the village. Asriel instead died, driving Chara over the deep end completely, essentially turning them into a mirror of Flowey/Asriel, but having more of an original intent to kill built-in.
So Chara and Asriel both went over the edge, but Chara had more trauma to start with, so they went more overboard and became irredeemable.
I should probably mention that I have only read a summary of the genocide ending, so I'm not super familiar with Chara, I just want to at least interpret that they loved Asriel at least, but the darkness in their heart corrupted them to a point of no return.
