Gaster took Papyrus' words into consideration.
Then suddenly, a memory came back to him. From a time before Papyrus or Sans.
"Hey, you alright?" Gaster asked to his friend.
"...no." Asgore replied. "Being blunt with you, no I'm not."
"What's wrong?"
"A human. Well, a group of humans." Asgore said. "They started picking on us because of our race. Called us livestock. Animals. They even tried to attack Tori."
"...what?" Gaster recoiled slightly in shock, his pupils vanishing. "Where do they live and what are their names?"
"Gaster, no." Asgore said, gripping his shoulder. "I don't want you getting involved in this. People are jerks. Being an ass to them won't change anything."
A few hours later, Gaster had been walking the streets in thought when a group of humans came up to him.
"Hey, bones. Where you going this late?" the leader asked.
"For the record, it's only 4 'o' clock." Gaster had responded before turning back around and starting to walk off. That's when one of the humans had grabbed him and thrown him into a wall.
"Time for you to return to the grave, huh?" the human said, a smug grin on his face. "Yeah, we'll make sure not to open your coffin once you get back home. And tell those goats we said hi."
"You're the ones who...?!" Gaster narrowed his brow, and a single bone came out of the ground.
He sent it right through the human's shoulder. The human screamed out in pain.
"Leave. NOW. And stay away from my friends and I for the REST OF YOUR SORRY LIVES." Gaster said, his purple eyes glowing. The humans ran away in fear, the leader stumbling behind them.
They passed someone on their way out - someone Gaster recognized. It was Asgore.
"Gaster... what did you do to him?" Asgore said, eyes shaking.
"I... I delivered justice." Gaster said, walking away. "I know you didn't want me to. I'll try to hold back next time."
"Gaster..." Asgore replied solemnly. "Mercy is something important. I hope you understand that. He deserved it, yes, but that doesn't give you the right to just-"
"I didn't kill him." Gaster said, teleporting away.
"...some people just aren't as nice as you are, Papyrus." Gaster said, a twinkle in his eyes. "You can live by that philosophy. I guess you were spared from my wrathful side."