I'm no fan of greenshirt, but his connection to that murder wouldn't (afaik) disqualify him from jury service, and I don't see why it would make him more likely to take a lenient view of the defendant either. We have no idea what his view of that horrific crime was. Having had that experience could have made him interested in taking part in this programme, I suppose.
Regarding the case, as a layperson my inclination was towards murder simply because he stopped and got the hammer. But if I were a juror and had the benefit of hearing all the evidence and receiving guidance from the judge, I might well have said manslaughter with loss of control. As viewers we can't really tell what our verdict would be without all the relevant information, and I think the programme makers could have done a much better job of including that information to help us viewers to form an opinion on how well or badly the juries performed.
I know the show wasn't concerned with sentencing, but after discovering that the real defendant only got 7 years (presumably served something like 4?) that felt incredibly lenient for taking someone's life in such a brutal way. I looked up the sentencing guidelines, and for the defendant to get a heftier sentence he would have had to met high culpability criteria, which he didn't (things like use of a firearm, concealing or defiling the body, loss of control in the context of other criminal activity, weak qualifying trigger) and/or there would have had to be aggravating factors. He appeared to have one aggravating factor (use of a weapon), but then several mitigating factors (no previous convictions, remorse, good character - possibly history of abuse of defendant by victim). He also appeared to have cooperated fully with the police, didn't attempt to conceal the crime, admitted straight away that he had killed her. So with all that in mind, if he was judged at medium culpability (starting point 8 years, range 5-12 years), the sentence was only reduced slightly from the starting point of 8 years.
That doesn't seem like a hefty enough sentence for killing a person. It's not like it was an accident. Her poor family 😞