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This was probably the dumbest to date
the cops would easily find the drug in the victim belongings, and after checking that every victim have the same product making 2+2 would be the logical next step
South Park explanation to human combustion was FAR better, almost 20 years ago
the only way the cops wouldn't find the drug out would be if the self immolation only triggers a while after the last dose is taken, so many users would have thrown the jar out, and their bodies being burned or ash would make it semi impossible to make drug tests