Haruki was in love with Robin. He has always liked guys (only or too, who knows) based on his feelings, longing, and actions towards Robin. Suddenly seeing how much of a creep and manipulator Robin is shocked and confused him, leading to the apparent lack of resistance. Even if Haruki did resist after the true colors came out, the size of Robin and their position would've made it very hard to escape or defeat Robin, not that was even a thing in Haruki's mind because of the shock.

Robin needed to be truly evil in order for Haruki to have the chance to like Fatal Diver. If he had died, Haruki would still mourn and miss him. If he is a good guy, Haruki would focus on Robin and ignore Fatal Diver. If he is a scumbag, Haruki can move on and get with Fatal Diver.