I found Cold in July to be a surprising, well-crafted movie. Except I cannot figure out why
Price and his men tried to stage the accidental death of Russel by injecting him, pouring alcohol on him and leaving him on the train tracks.
At this point in the movie:
Russel believes the body in the grave is his son.
The police have ample evidence of his new crimes to put him back in jail.
He's in local police custody already (so it means they caught him).
Given this, their actions seems uncharacteristically random and, frankly, risky given the chances of it going wrong against them. Based on all the reveals later in the film, it seems to me they've already covered their bases, so why do they take that action, including falsifying a cover story about Mexico?