I think what slow motion does is reveal the full violence of an action. Some commenters have mentioned that slow-motion tackles and punches look much worse than they do in real life. However, in real life, I think we underestimate the violence. Medical research into things like CTE has revealed that there is actually brain damage occurring during those plays.
Notice the research used a gunshot video. I think the average person underestimates the violence inflicted by a bullet because our human intuition fails us for small objects traveling that fast. I think what slow-motion does is help reveal the true violence inflicted on another person whether from a punch, tackle, or gunshot.
Update:
IANAL but also note that in this specific case, murder during commission of a felony ( armed robbery ) would be classed in many states as felony murder and would be first degree murder. See http://criminal.lawyers.com/criminal-law-basics/murder-durin...
So if this is a felony murder state, the slow motion would actually help the jury come to the correct conclusion.