Is the murder/dismemberment of the guy in the stopped car true?

It got me thinking. Ryder truly intends to kill Halsey and does kill Nash in a pretty gruesome fashion by tearing her in half wich would reinforce his anectode. Logistically however he didn't seem to be carrying a good enough knife to do the cutwork. If that's the expectation, that it's the same knife he threatens Halsey with, that's a stiletto wich wouldn't even cut at all. Those are just for poking.

But with any knife really... i would imagine it would be some serious work. In a weather where it's unlikely anyone was travelling in light clothing (so he'd probably have to undress him), with the body either laying in a crammed VW Beetle or on the roadside in heavy rain. It just seems implausible that Ryder could do it all in those conditions.