Ralph admitted that he hit Colbert over the head at the factory site and that he thought he could make it look like someone ambushed Colbert. So why did he bring the body back into town?
shareHe didn't mean to kill him he said in the final confession/interview scene. Meaning he probably brought him back to town feigning helping him from their "attacker" where he could be a hero of sorts...until he realized he was dead and out he went.
Gr8tScape
Firstly, the guy wasn't the sharpest tool in the box, so what makes 'sense' has to be understood from his perspective. His plan was to make it seem like they both got jumped. It isn't explained but my guess is he would have driven the injured Colbert back into town with that cover story.
Being that he killed Colbert and he still needed to get back into town...he has no car, he adapts his first plan, taking the body back to town and dumping it. Beats me why he leaves the wallet with money in it, or why Colbert would want to show the idiot a block of land with some surveying stakes in the middle of the night in the first place tho, but there are a lot of plot shortcomings in this story.
While I'm here, this is a decently made film with a flimsy plot and some stereotypes that stink up the effort, and it would mystify me as to why some people consider this a classic if I didn't have such a poor view of humanity and thus already know why people mark it above its objective merits.
I'm not one to see gay subplots everywhere in films but when I rewatched ITHOTN again recently I thought maybe Colbert took Ralph somewhere quiet to hit on him and Ralph wasn't that kind of guy and took the opportunity to knock Colbert out and take his money. "Queer rolling" is what this was called once upon a time.
Why else would a leading citizen want to go somewhere remote late at night with a low-life like Ralph? Plus Lee Grant's character comes across as a shrew. Maybe Colbert wasn't getting any at home and Ralph looked good (oof, what a thought).
Leaving the wallet turns out to be a great idea. If it wasn't for Tibbs' influence over the matter Harvey Oberst would have likely have been tried for murder. Even if the truth of the matter came out in the trial and Harvey was let free it would likely be a very cold trail and Ralph would have gotten away with it.
As far as why the car was left where... maybe for the same reasons, misdirection. If it wasn't for Tibbs the local PD seemed to be under the impression that the body dump was the site of the murder. There's a number of things I see discussed on these posts that makes you wonder what the layout of the area was really like. Maybe the diner was within walking distance of the factory site. Being in the middle of the night that would make Ralph a key witness if not a suspect.