Imagine a long hallway segmented with closed doors, where each door represents a decision made during a mission. As you continue walking down the hallway, opening doors, the morality of each decision becomes more difficult to square with your original frame of reference. Finally you encounter a door that you decide to leave shut.
That is roughly how it works, self selection. The SF groups that, to put it charitably, operate under legally ambiguous circumstances are people who know each other really well, and they only invite in those who are a known quantity. There isn't any kind of laboratory-prisoner-execution/choose-the-red-pill sort of test, the evaluation occurs continuously in the daily performance of one's duties.
SEAL teams are, relatively speaking, high profile SF - and therefor much less likely to be tasked with strictly illegal and morally objectionable missions. I have a lot more trust in someone who has SEAL team affiliation towards the end of their military service. They've walked the hall and stopped opening doors pretty early on, so they're a well measured quantity.
Also, it is my experience that veterans with combat action ribbons are far less likely to mindlessly appeal to state authority or be swayed by utilitarian justifications. Of the dozen guys I've kept in touch with, most are libertarians and a few are anarchists. I guess seeing the ugliest side of the state will flip a switch :)