Friendly fire is never acceptable! The LEO should have identified the target—which he did not! He broke a fundamental rule in firing a weapon.
Keep in mind it was a RIFLE. It may very well have passed through the intended target first. A rifle round can go through three bodies before finally coming to a stop. It can hit bone and change trajectory and continue on to go through a second body.
There needs to be more detail about the trajectory before this can be rightfully judged as an accident or neglect.