Okay I know this is a long shot, and I know these case connections between different Unsolved Mysteries segments are rarely very likely, but hear me out.

Here is the composite of Don smith's killer, the man who called himself Larry Monroe:

https://unsolvedmysteries.fandom.com/wiki/Larry_Monroe?file=Don_smith_larry_munroe.jpg

Now, go look at the original sketches of the Thompson killers. (Sorry I couldn't find an image to post a comparison, the original composites of the Thompson killers not the new ones), and there looks like there is an eerie resemblance to me to the one on the left. The Thompson killer also was seen only from a distance, so any differences would certainly be understandable, but to me there is an eerie resemblance.

Also the verbal descriptions are almost identical:

Larry Monroe: estimated height about 6'0 180 lbs
Thompson killer: 6'0 180 lbs.

Date of Don Smith death: May 11, 1987
Mickey Thompson murder: March 16, 1988
Less that one year apart.

Also, Hemet, CA where Don Smith lived, and Bradbury where the Thompsons lived, are less than an hour and a half apart.

Now granted, we don't know exactly where Don Smith picked up the hitchhiker calling himself Larry Monroe, but the relatively close proximity between the two victims is pretty stunning. Also, Don Smith's killer was described as having relatively light skin for an African American, which was not said about either of the Thompson killers. But remember, the Thompson killers were only seen from a distance and were booking it away from a crime scene on ten speed bikes.

Also, hitmen are often drifters who want to make big scores rather than find honest work. That was obviously the case with Larry Monroe. I don't think it is to inconceivable he could have been a small time criminal who hooked up with Michael Goodwin and graduated to a big score contract killing of the Thompsons within a year.