At least it will calm people down for the time being. The way this is going the longer it is drawn out with no arrest, the more potential for this to get ugly. Arrest him, so the family can at least go through the judicial system and have their day in court. Then see how much it calms people down. At least they will be able to see it play out in court, unlike what they are seeing now, the police making the decision because 'there is no evidence'
It's not up to me whether they arrest him or not (and it's not like I am going to be sorry for GZ if they arrested him), but I am not sure they should arrest people based on public pressure.