Just be an ordinary elephant making a monkey out of a donkey who thinks it's too funky to be a donkey. Kiplingesque but at the same time not unKafkaesque. The categorical imperative, in other words. The argument might hold up in court if the yahoo had a good lawyer. All that's known at the moment is he was followed by a suspicious stranger who bought him lunch.
Nobody can positively identify the stranger.
Restaurant employees either didn't get a good look at him or said he could have been anybody. Robbery doesn't appear to be a motive. Nothing appears to be a motive. This may turn out to be an impossible crime to solve. The perfect murder, so to speak.