Ho, for fuck sake. Do you really think I was possessing Frisk when he decided to kill all monsters in the ruin? Of course not, there was not enough DETERMINATION to bring me back to life at the time.
True, I did influence his later decisions but he was still in control, I only assumed direct control against sans because he could never have beaten him without all my power.
Frisk took the decision to summon me with his own will, like the big kid he is. I can only do what I do because he allowed me to.

Poor Frisk. They're canonically pacifistic and all-loving, but now literally can never live it down because up to two sinister forces have come in to force them to murder everyone and sell their soul afterwards.