Razz,
I think you will find that it is often easier to prey upon the weak than it is to take from one with the same intentions as you. As to your observation about Flagg, I think you're correct. He does tend to prey upon thise who are weak rather than those who are evil. The weak are easier to guide and less likely to throw off the yoke.
DT7 Spoiler:
Spoiler:
They are also less likely to eat you alive.
I also noticed this trait was present in Leland Gaunt of Needful Things. He preyed upon people's desires and their obsessions, which made them similarly weak. Check out Linoge in Storm of the Century who worked through the fears of a parent for their child. Your observation does seem to be a running theme within King's worlds, much as it is here in this one.