@Exkalamity I'd imagine she shot him right at the edge of the rooftop, and the momentum from the bullet made him fall forward. Then, he landed head first, masking the gunshot wound with the impact damage. Since it was a sniper rifle, the bullet could easily pierce through the head and landed somewhere unsuspecting.
Or just don't think too much about it and enjoy this as a gag manga.
@CookieMuncher so its already canon that she is a psychopath 😂
@MagnaDrake I think thats what happen, but because she use high spec sniper rifle in pretty close range, the bullet can easily penetrate the cranium and hit the wall to leave a bullet mark. Its an evidence that cant be erased easily, so its funny that the forensic cant find anything regarding this though.