William Afton:

Lost his family, got spring locked in suit, trapped for 30 years before being burned several times.

Ink Demon:

Deemed as a failure before being locked up by Joey Drew and then being tortured and experimented on by Wilson Arch.

Cyn:

Had her free will being taken away form The Absolute Solver before being used as a puppet to destroy planets before getting killed by Uzi

Chara (genocide route):

Tried to free monster kind only resulting in both their and Asriel's deaths. Chara would then become a ghost to see all of monster kind being killed by Frisk

Seong Gi-hun:

Played squid game which traumatized from him seeing his friends and various other people died. He tried to put a end to the games only to be defeated by Front Man