"I forgive you"
After everything he had done to get here, after everything he had failed to feel over the years, even by his father and mother's side,
Asriel was stunned to hear the kid he had tortured repeatedly, who he had used and betrayed, utter such words.
He had been a kid himself so long ago, but after his soul was corrupted and reborn in a twisted way inside that flower carcass, he was unable to feel anything, good or bad, any longer.
And so corruption and sadism began to take root.
Now, all he wanted was to destroy and ruin the world around him, and yet Frisk was telling him he forgave him!
"What? I need no forgiveness, you fool! Don't you see I already took the soul of every single monster! My power knows no bounds!"
"No, all I see is a scared little boy who needs to rest at last, who misses receiving a sweet hug from his mom and his dad and his friends" Frisk explained,
finally understanding the reason behind Flowey's, and thus Asriel's, hatred and anger: He was afraid, unable to experience the true feelings he had as a living boy. "So I forgive you"
Asriel was ready to scream again, to murder Frisk and take his soul as well until he realized he was sobbing.
He was actually feeling something! He was actually needing something instead of just craving chaos.
"I... I'm crying" He mumbled, confused.
Frisk rushed toward him, and though for an instant Asriel feared it had all been a trick and he was about to be murdered, this never happened: Instead,
Frisk gave him what he had been waiting for more than anything else in the world: A hug.
"You are free to go now... rest" Frisk whispered, finally setting Asriel's soul free.
In a sudden flash of light, Asriel was set free from his torturous existence.
All the souls he had collected were released and brought back into this world, reestablishing order and finally allowing a sad little boy to rest as he always should have.