Seraph of the End: Guren Ichinose: Catastrophe at Sixteen (manga) 4

What would you sacrifice to save the ones you love?

Guren just barely escapes his first encounter with the disturbed product of forbidden experiments and one of the terrifyingly powerful vampires whose ire they’ve roused, but the battle takes a heavy toll. It leaves him with a permanent stain on his soul and more questions about what exactly his childhood sweetheart, Mahiru Hiragi, is after. And what does her prophecy about the world coming to an end actually mean? Before he can get any real answers, however, Mahiru begs him for an unthinkable favor: to kill her by his own hand. 

But war waits for no one, and soon Guren’s friends and personal guards come under savage attack. With no time to spare and thousands of lives in the balance, Guren must measure love against madness, and the survival of his friends against his own humanity.