I’m calling the missing person here Brian not “William” because his mom calls him this.
This YT clip is a difficult thing to watch/listen to due to the technical issues experienced during filming, however I think the mom didn’t really know her son as well as she thought. No parent wants to concede that their child may not be as pure as the driven snow, when plenty of the indicators suggest otherwise. All involved here seem to be wanting to paint a pretty tame picture of everyone else but Murphy (including the coworker who says he only listens to church music and couldn’t possibly be involved in making rap videos with Brian etc). But it seems to me that there were possible drug addictions (in the case of Brian, he was apparently on prescription “medicine” as well as using “recreationally”), some dark and dodgy characters and relationships, perhaps some sideline “business” activities etc going on that Brian was a party to or at least knowingly on the periphery of.
It doesn’t minimize the seriousness of the situation, the fact he’s still missing, the likelihood that Keenan Murphy is very much involved with his disappearance, or the mom’s pain. But in order to solve something like this you kinda have to examine the circumstances in a clinical and unvarnished fashion rather than glossing over the stuff that might reveal what was really going on in the lead-up to Murphy leaving that porch in a vehicle with Brian, as is alleged.
The “friend” Shaun claimed to the mom that Brian got “stuck on” something backing out of the driveway in his Jeep, but after looking at the fence and driveway the mom couldn’t see how this could’ve happened or what he could’ve possibly hit. That same friend - while “helping” the mom search for Brian before law enforcement got involved - kept “looking down” into creeks as they drove around looking for him.
IMO: taken all together, I wonder if perhaps Murphy (who was on the run and likely needing a vehicle for escape or to turn into quick cash) was attempting to steal the Jeep while Brian was distracted and smoking drugs on the porch. The Jeep is probably why Brian was invited over in the first place. Brian tried to stop this but ended up getting shot, hit or run over by the vehicle (probably his body was what the Jeep got “stuck on” but with Murphy behind the wheel). Then Murphy - with or without assistance - disposed of him and told Shaun he was already an accessory to multiple felonies and would be killed next if he ratted.