The thing is, Trashcan man was manipulated by God all the way through his treck in the desert looking for weapons, just like Harold Lauder was manipulated by Flagg in Boulder. Lauder wouldn't have been as bitter and evil as he became without the manipulation of Flagg, and Trashcan man wouldn't have taken the atomic bomb back right at that moment without being manipulated by God himself. There are so many chapters where we follow Trash that explain motives he can't even see himself!! God manipulates him through the love and devotion he has for Flagg just like Flagg manipulating Lauder through his perceived betrayal of Franny and Stu!