Runner Runner Question

I am playing 100max nl in Windsor. Sitting on about $250. I have 8s7s in SB, UTG(just sat down) raises to 12 preflop and it is called by 2 players including me.

Flop-

2s 4s 8d

i check knowing that this could not have hit the raisers hand and hoping he will bet out. He does, he makes it $25 its to me and i raise to $75. He goes all in for a total of 100, 25 more to me i call it. He has Ad 10d. he hits runner runner for the flush. So im like wtf? there were like 3 other people saying it was a good play on his part. he says he is good if i only had 4 to the flush, why doesnt a set slow play enter his mind? How can this call be justified? not only by one player but by 3 others?