I like to think that Frisk is trapped inside their tainted soul, so they try to get Chara to kill, so she can merge her soul with Chara's.

And I'd like to think that when Frisk were to posses Chara, that she'd kill everyone in secret.
Lour them away inconspicuously.
Wash the knife.
Wash the dust from your clothes.

And just play dumb. I also feel like (unlike the real Genocide) Frisk would keep up the good guy act. And only when everyone were about to reach the surface, she would be a little brat and let her murderess desires be fed.

But that's just my opinion^^

I've also grown a liking to a smaller, cuter, design of drawing. Chibi..right?