The Ninja Epidemic

You glance around in search of the third ninja and find him preparing to sneak up on you from behind, the bastard. Without thinking you throw one of your sais at his head. Surprisingly you connect. The ninja stands there in shock for a few moments with the sai sticking out of his head before he falls to the ground. Your shocked that you killed another ninja so easily. Shocked even more so that your notoriously horrid hand-eye coordination for once came through for you.

You turn back to face the other ninja and see that he is running away. Do you give chase, or let him go?