So maybe one reason Igor was not shot like Tim, was that he was driving, and shooting him would likely cause an accident, damaging the truck, and calling all kinds of attention to DM and MS. In the Bosma test drive, DM made sure he was driving. It was also later at night, with the cover of darkness, and in a more rural area. DM likely thought they had a better chance of getting away with it, and as mentioned before, Igor was a very large guy -- difficult to move the body around, and into the incinerator with truck seats. It is all so gruesome, and DM was of course smiling' while MS was doing his best to be invisible. Maybe DM even liked the sounds of Tim's truck better, and wanted to see it before making his final decision about which truck to steal.