It means he's going to continue to torture him (in this case, flicking hot ashes on him). The title "ashes on the head" refers to a funeral rite and represents grief or mourning. I think Daichi is mourning/grieving that his ideal guy turns out to be a sadistic freak, but now he's trapped by his love. The suffering begins....
The title is actually a double-meaning: ashes from sadistic sex and ashes on head meaning something has died and one is grieving/suffering from loss.
I was thinking the double meaning was referring to when he said, "I can live on with those memories". Tooru sounded very fatalistic, like what he was about to do would be the end of his life so I imagine... maybe he's a serial rapist or even murderer! Ashes on the head could be Daichi's cuz he's gonna kill him or on Tooru's cuz his life as a normal person is over/already gone
I don’t get the ending… i get that Tooru wants to said that his life is fucked up. I get the twist . But the last panel means what???