IMHO it's a no brainer that Lacco commited this crime.
Amongst all the powerfull evidence against him, is the statement he made a month prior to passengers in his car, "there's a body going in there"
Coupled with the sand found in his car, he could have readily dug the shallow grave prior to abducting his prey, and have the body bag ready.
If it hadn't of been for hijacking (Lacco's words) the deceased's car and, damaging the sump leaving a trail of oil, the body may never have been found.