I'm not even 100% sure the people that got turned into the sentinels, did so willingly/knowingly. The process seems to be "go in a green tank of goo" and ends with "you won't remember you went through that process." That feels ripe for grabbing people and doing it to them. This is one dark fucking story, if that is the case.Also from what was implied from the comments in their OZT conference call (and Bastion's reveals to Val), I don't think many of the other parties really knew what Bastion was doing / planning.
I wouldn't be surprised if his entire pitch to them was an intentionally vague "Mutants numbers are increasing and you need a means to respond if things go south. I'm building a next generation of Sentinel so join me so we can protect humans." The end result was still no less awful though