DNA Wrong

I believe he was the killer, the bar scene clinched it for me
I think the new Chief lied and didn’t want to give Dixon ( who he hated) the credit

He was a mean man and from the bar scene we hear a rapist too. But his DNA did not match and what is more important, he had an ironclad alibi putting him outside of the country at the time of the incident.

I rather see it as though this guy is a bad apple. The first shop scene establishes him to be a mean man, looking for trouble. And it seeds wonder in our minds that he may be... The bar scene then cement him to be a rapist too... If we would only have the bar scene, we would all wonder if he was just blowing smoke to a friend and trying to sound cool... Since we have both scenes, we are easier to accept him as a bad apple who deserves what is coming. So the two scenes are necessary in order to justify to us the audience that the final road trip is a satisfying conclusion to this story.

I don't believe he was the killer but I believe he was there that's why they didn't find any DNA

The movie wants to leave this in a state of uncertainty, but this is not satisfying. Much of the movie is focused on finding the killer. But any discussion of who was the real killer is sort of a moot point.
But, it doesn't make any sense that the guy in the gift shop would be the killer. There is no indication that he's a moron. As such, if he were the killer, and if thus far he had gotten away with it, he would never go out of his way to approach the victim's mother and make those statements. People are dumb, but that goes a bit far. (And I don't get the sense that he was going to kill her. Broad daylight, customers coming in etc.)
His overall role in the movie is really odd. He is from Idaho (based on license plate), and if we assume the new chief is telling the truth (no great reason to believe otherwise), then this guy was in the military and was out of the country nine months earlier. If he's not the killer (as shown by lack of DNA and as shown by his being out of country), then why would he have such an affinity for the sheriff or such an interest in this matter? And what is he doing in the town anyway? At the end of the movie, Mildred and the former cop confidently drive to Idaho to find him, strongly expecting him to be there.
His actual comments about the burning don't make sense, even for a serial killer. (He suggested that the burning body turned him on.)
In the end I find him to be a distraction that does not quite fit in any sense.