Eh, *shrug* I say just headcannon what you want and don't let anyone try to change your opinion. ^^

Personally, Chara is a Chaotic-neutral character to me, being very messed up by the past I wrote for them. The laughing at poisoning Asgore thing was something they picked up from an older sibling who did the same(laughing at others' pain) and hating most of humanity was due to something their father did. All they wanted to do was help their new monster family, even if it meant killing humans. As for Chara appearing at the end of the genocide route, power corrupts and absolute power corrupts absolutely, being linked to Frisk's soul due to being their twin is very bad in a genocide route, as when one goes dark, so does the other one.

I could say more about my headcannon, but I'd probably just bore you XD But it's just my headcannon, it's not cannon like you said. It's only cannon in my own mind. Not gonna force it on anyone XD

I did have more to say, but I've lost the train of thought I just had and I probably won't go looking for it anytime soon. XDD