" Finicum was shot three times in THE BACK by the other officer who was behind him."

That makes no difference in this case, because Finicum was armed, and was attempting to draw his handgun.  In addition, officers can also use lethal force to protect others, and not just themselves.   The Oregon State Police officer had every right to shoot Finicum, in order to protect the officer that Finicum was facing, and posing a threat to.