I just remember the guy picked up what seemed to be an innocent, cute guy on the road and he sort of wanted to sleep with him which is why he picked him up.

At the end though it was revealed that the guy he picked up was in fact a serial killer, and I think the cute hitchhiker even had a severed head in a bag (?)
Anyone know the name?

It's To the Sea by Asada Nemui.

Thank you!