Mahou Tsukai no Deshi ga Warau Toki.

Shou Takahata is stuck between a rock and a hard place. Facing imminent death having stumbled across the world of magician's, she's struck a bargain with one such magician: to befriend his apprentice and become "dear" to him. The main problem with this arrangement? The apprentice in question can only feel emotion when he slaughters those he holds "dear"! (Source: Yen Press)