There's a difference between people with a predisposition for sociopathic behavior who do fucked up things like this regardless of their circumstances, and people who get put in fucked up circumstances and act like sociopaths.
Well I think Random Probability was saying that under the circumstances of Mai Lai, anyone normal psychopath, average sociopath, or what once considered itself a normal sane and moral human being... may well act in the same manner. As for the difference you're claiming... I agree that there obviously is one, but I'm not sure that its as large as many people would like to think.
As for the Marine in the video... he may have been a perfectly normal guy 10 years ago... he may have had a pet puppy that he loved as much as you and I would. If could be that X number of years of being trained to kill, being conditioned to accept death as acceptable and seeing comrades die (assuming that this guy actually served in combat)... may well desensitize any of us to the point that puppy flinging might register as teh funnay rather than horrific. We don't know what situations that Marine has been in, we don't know if he saw havlf his platoon fly through the air as an IED shattered bones, shredded flesh and took lives. We don't know that this guy didn't end up on the clean up crew after a particularly nasty suicide bomb in a crowded marketplace. We have no idea what he's experienced, who he was before he experienced it or what may have led to his puppy flinging behavior. That doesn't excuse the behavior... not in the slightest. We are all responsible for our own actions.
However, it may mean that we aren't able to meaningfully shove him into Box A (sociopath), Box B (psychopath) or Box C (Normal Dude that got his brain fucked by war)... which leaves us only with Box D (WTF?).