Ok so here's the deal, I've been thinking about Frisk from Undertale a lot lately, namely how they must've felt when they fell down underground for the first time. Imagine falling down a giant hole and waking up in an unfamiliar place, only to find that pretty much everyone you come across is going to try to kill you. If that wasn't enough, imagine going through all that and basically not being able to trust anyone you find because you know eventually they will try to kill you, just like everyone else. Not to mention you have no contact with your parents or anyone else on the surface. It's a darn good thing Frisk is so determined, otherwise I think they would've given up halfway though the ruins!!
Now about Chara. So let's first imagine their situation. This random kid falls on your grave, and starts killing everyone in sight. You initially posses them to try and stop them, but eventually your LV gets so high you can't feel anything anymore except the urge to kill. And really all you want to do is see your brother, but you can't because he's stuck as a flower in the particular timeline you're in.
Back to Frisk. Even as determined as they are, there are probably times they want to give up, as they are still human. I believe the genocide route only happens because of Frisk's deepest desire to give up and stop trying to free the monster world. That's when Chara possesses them, tries to stop them, and either succeeds, or fails.
This was longer than I intended.
sorry.