I think he got replaced by something else entirely.

Probably because the spirit know what Kevin likes, so when the impostor got it wrong they knew.

Open a “get rid of the person you hate without leaving any evidence” and get rich while avoid being eaten.

If you think about it, there were 6 tickets 6 passengers, which meant that the driver was the only sus one since driver doesn’t need to swipe tickets.

At least he actually kept you safe, some other shapeshifting cannibal asshole would have let you died first and then ate their kind second. And he even paid you handsomely too, though it is better that you start managing your finance now that he(it) will move away in a few weeks.

I think one of the common explanation is that he got them from the people he killed.

Could this be some sort of memetic hazard and the scientist purposefully locked himself in there prevent it from spreading.

Just a mildly unpleasant side effect!

But it worked as intended didn’t it? They lost all of their weight in no time you know.