Keigo knew Touya when they were younger. He knew Touya was abused by his father but didn't know who his father was. He ends up coming to the Todoroki house because he forgot something work-related to tell Endeavor, and he's led to where Endeavor is, with the shrine. He realizes Enji was Touya's father, who abused, killed his best friend(or at least, killed who he used to be) and everything crashes down on him all at once.
And Keigo witnesses the death of a hero more ways than one that night.
My mom has gone from 'yes. i suppose i will indulge you children and get the dog. only to guard our family from bad. i do not like dog.' to 'you see this dog? that's my boy. my son. i would die for him. i will kill for him. you can't stop me'
WANNA SEE ME EAT THIS WHOLE TUB OF MAYONNAISE ? hHHHAMAMAHHHHAHHAHAHSHHHRHRHJHGHGHGGGGGGGGRGGGGGGG, HHHANANAHHHHHHHHMMMMMMMMMMMNMNMMMAMAAAGGGMMMMMMMPHTH;;;;;;::;: HOOOOOOOOOOOOOORRMRMRNRNRMMMMMMMMMNMNGHHGGGGGGGGGAMMAMNNMMMMMMMMMMMMMMMMMMMM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!