I agree he probably never considered the LE asking for DNA swabs. The thing is, if people in the neighborhood decline - perhaps because of civil liberties violation, etc. - he could have also declined without standing out. The cooperation of the neighborhood pulling together to find one of their own was his undoing, imo.
I do think he would've been caught anway. LE could get DNA samples without asking. If there were beverages served at the vigil, for example, they could retrieve his discarded cup or bottle.
I never thought that he could be a hero finding her, but that could've crossed his mind. But, I bet the LE would be looking at anyone in the neighborhood who "found" her. Didn't they ask the public NOT to look, but to let the trained people do it?
But, yeah, I'm sure the knock on the door Monday was indeed distressing to him - he really was distraught.
Right! That is what I mean...he never thought in advance about the dna swabbing, and he never thought that they would not want any untrained people searching for her, for fear of tainting the crime scene. Especially never thought the FBI would be there what, the next day or two?? That was some swift action and bravo to all of those that found her and gathered evidence so quickly. But anyway, yeah I don't think he realized that this would come apart so quickly for him.
So if he had envisioned being the hero, I don't think he would have thought about anything like being a suspect or them getting dna samples from other things. If being a hero was a factor, and I am just speculating, then I think that would have become his main goal and he would have lost sight of the details that would prevent it from happening.