I question why even move the semi? Why not just take what you wanted, the person or their possessions and go?
Obviously was not robbery. Wallet and cash were left behind. A 53 year old, thin truck driver would not be my first choice for some sort of "human trafficking" need. I mean, lets face it, truck drivers just tend not to be in prime health because of their job and lifestyle. The stress and meals obtained are just not conducive to a healthy lifestyle. Respect them even more because of this btw!
So...Why?
JMO I conclude that only 'the thing' the perp wanted was to get DS out of the picture.
They didn't want his truck. They didn't want his load of livestock - I suspect you'd have to be in a very niche market to know what to do with a 100 baby pigs and plenty of road traffic cameras would spot your journey!
They didn't even want his $2,000 or his debit card (in my country you can 'tap and pay' £100, approx $120, per transaction in stores with no checks on a debit card). They didn't want his ID papers or heavy vehicle truck driver licence, or his mobile phone, etc. Because they wanted nothing of his about their person IMO, because they know him and they knew they might likely be searched / questioned.
What else could they have wanted? To drain / syphon his expensive tank of diesel or gas - we know this is of value? Was there any sign of that?
I assume the perp didn't want his body for nefarious reason in the same way as a woman would be kidnapped and kept alive for abuse but since his body is unknown location I suppose it cannot be ruled out.
JMO but this was 100% personal. He was either targeted by someone he knew very well, knew his route etc and lay in wait in a particular spot knowing he'd be slowed down at a turn off, luring him to stop on a false pretence. OR he stopped to help a friend in need or someone who was genuinely in trouble and in the process, they got into conflict the one lost their temper and took him out.
They drove his truck slowly but proficiently, insofar as it didn't come off the road into the verge or such, just far enough away from the original incident that they could walk back to their vehicle 3 miles back, a very walkable distance and doesn't leave their vehicle abandoned too long. If they're local with good knowledge of the roads, maybe they also put it on a road that had far less passing traffic and would be way less likely to be noticed for many hours to come so they also 'bought time'.
If they understand how Weikmann's works then they know that the piglets won't be missed until operations begin in the morning (what time 0800?) and even then it'll be a little while before anyone identifies that it's DS's 100 hogs not in the pen.
It's either deeply personal, someone who wanted DS out of the picture (for example business competitor), someone he knew so well that a huge argument turned into a rageful murder, or total random lunatic did something inexplicable. My argument against random lunatic is they usually target the vulnerable or 'easy prey' and someone in a huge truck is not that. Maybe random passing gangsters needed help with their vehicle, he provided it, they said thanks for that, shot him dead, ditched his body away from the scene of the shooting?
JMO MOO