I don't think it's likely that Dell is to blame. Probably someone made an extra hundred dollars per computer through straw purchases that were resold to NK. If we ignore the detail that Dell management probably doesn't want to wake up to the sight of a mushroom cloud outside their bedroom windows, there is the detail that they probably don't want to wake up to the sight of the FBI arresting them. The simplest explanation is a straw purchase, and that is what I will assume in the absence of evidence to the contrary. This is not like a Cray, where the company would know for each individual purchase who wanted the machine and why.
There is a huge difference. I would no more blame Dell for "aiding" NK than I would blame Bic if we discovered that their pens were in use for the NK WMD program.