the suit sans wears is actually inspired by michael jackson white suit he can be seen wearing in the smooth criminal music video

so anyway for some much needed lore
so long ago two species ruled the world yad yada yada
FELLNEGATIVE RF-TAKE

the underground started off like underfell

all monsters were violent and just horrible people etc etc

but one day asgore ordered a full on civil war between every single monster, because he hates weakness or something plus there was a lack of resources
everyone was for themself, there were no teams no nothing
the monsters were basically free to do whatever destructive things they wanted
they attacked, they looted etc etc

sans was one of the survivors (like the whole main cast survived)
and he happened to be one of the people who was able to take the most
he became rich and a total snob
he views himself as better than most
and after the underground achieved temporary peace due to having enough resources and leaving only the most powerful monsters alive
he started viewing himself as better than being violent and actively avoids all violence due to his arrogance he also simply doesnt want to get his outfit dirty