Shounen Psycho: Ani ga Boku wo Koroshitakute Komatteimasu

Reishin Amo's life turns around when his mother marries into the wealthy Kakurai family. But it turns out that the Kakurai family are hitmen by trade... And his new stepbrother Garaku is a psychopath who kills for sport! Now Reishin and his sister Sarasa are his targets...

(Source: Coamix)