Setting aside all aspects of accuracy of story or landscape or props or whatever, and any problems with boom mics this is plain and simply just very bad. The script is the worst offender, truly awful, after Charlie's first murder when he tells Caril-Ann that he did it for her I couldn't believe the response. I actually mouthed to myself verbatim what a corny line would have been in that situation but she actually said it. For a film dealing with a serious subject with a purported serious tone was the sheriff's assistant comic relief or just atrociously written. The car-chase scene near the end was pure farce.
This film is lazy on many fronts but none worse than the character of the mentor. This is the sole justification for why Charlie Starkweather would embark on a killing spree, although at least this is more justification then for why Caril-Ann would go along with it. This seems to me the biggest of corners cut to tell a story, surely there must have been more to Starkweather's background than this, and how accurate is this sub-conscious cloaked man as a part of Charlie's personality anyway? I think better analysers than me will be able to give a whole load more reasons not to see it, all I can do is tell you it's bad, very bad, and despite the occasional flirtation with so bad it's good, it doesn't even merit that.