so I’ve got this personal headcanon that chara has the very tattered, worn remnants of their human soul still in their possession, and at the end of the two main runs they use the little power left to them. In a kill all run, they recognize that you, the player, as a human, are evil, and that they were right the entire time about humans being evil and they use the little power left to them to overpower Frisk, who’s lost their determination in the face of such great tragedy and their desire to destroy humanity is, from that point, hardened and immutable. 

But at the end of a true pacifist, they see Frisk’s kindness and they conclude they were wrong- this is not an evil human, so humans, by merit, are not innately evil, and all that hatred and desire for vengeance drains away- and that last moment when Asriel screams JUST LET ME WIN! and shoots you, and your health drops to that amazing .0000000001/20? That’s Chara, using the little power remaining in their fractured soul to shield just a tiny spot of yours to keep you alive, and together, both tiny fractured pieces of determined souls that refuse to kill or be killed, you save Asriel. 

Supplementary headcanon is that Chara’s soul stays behind in the Underground with Asriel’s soul once he reverts to being a flower and they play in Asgore’s flower garden again like they used to.