If you want to play armchair shrinkologist, the way to go is to understand that their entire ego structure, the story they tell themselves about who they are and how they fit into the world, is at stake and, as they see it, to lose would be worse than death. So they HAVE to fight for their version of reality. It's life or death for them.
If I were still teaching interview techniques, I'd have the audio off, and copy Slick's motions into slo-mo. He is a classic* sociopath who is having his world cave in around him.
The shoulders forward movement is threatening - more than just a bit aggressive. Reaching into Wallace's space like that was actually a bit of 'sleigh of hand'. Slick knew he'd be lying and wanted the hand distraction to give him some 'breathing room'.
He reflects the actions of a coward. He wanted to hurt Wallace, but is even too big of a coward to do it.
Wallace would be wise to have someone else start his car in the morning though.
*Full disclose: I'm not a licensed pyscoholgist, just a old street copper who did 20 without getting injured. ( Dagnabit, couldn't even claim disability - gotta pay taxes on my pension)