Did he think Arnie was killing the people? He seemed suspicious of him. True, there was motive. But what was he thinking about how quickly the car was repaired?
And did anyone else in the garage see it destroyed then repaired so fast?
Who even reported the crime in the first place?
Detective Junkins was suspicious of Arnie exactly because of motive as you say. Motive, opportunity, etc.
As far as how fast the car was repaired, Junkins must have simply believed Arnie's explanation that the stories he heard of Christine's damage was exaggerated - it wasn't that bad after the glass was swept up, shit wipes off, and the punctures were easily Bondo'd and painted, etc. Junkins probably didn't believe Christine was "alive" or anything else supernatural going on until the end of the movie.
The only persons who would have seen Christine restored from junk to mint almost immediately was Arnie and Darnell had he lived till the next morning, but Christine killed Darnell before he could have reported it.
Didn't Christine kill Darnell while she was burned?
shareYes she did.
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Yes, but Darnell saw Christine roll back into the garage as a crushed and burning hull, so he knew she was indeed severely damaged with no story exaggeration suspected. If Christine didn't kill Darnell, he would have seen the transformation the next morning along with Junkins and the State Police forensics team, so only Arnie or Darnell would have seen Christine restored from junk to mint overnight.