Due to the rise of defense contractors as major businesses only happening with the rise of industrialization, I assume this is only really applicable to the World Wars (maybe the U.S. Civil War?) and on.
Anyway, do the acts of defense contractors count as war crimes or anything else in the realm of illegality? Even if they do, is this often overlooked for the benefit of the global economy? Has this changed between the first defense contractors and the modern day?
This really depends on the conflict; almost by definition the victors are not really obliged to do anything specifically outside of domestic pressures, for example after Desert Storm there was no effort from the US to take Iraqi equipment back while after WW2 there was significant confiscation of German assets by the Soviet Union. You might need to be more specific on who you are talking about for a better answer.