one word explains all of this: methamphetamine. (and I say this as someone who believes in drug decriminalization or legalization)

I'm generally for (soft) drug decriminalization, too, but I don't have much experience with meth heads :)

Would someone upped on meth be able to conduct himself well via email, sending polite thank-you's?

Yes - my experience with meth heads is that they can be perfectly normal in their interactions, but once high on meth they can be aggressive, obsessive compulsive and destructive. Granted, this can't only be blamed on meth (or drugs in general) - the person was obviously sober at some point during this trip - or at least sober enough to know what they were doing. This all seems like a planned out event to have fun with a couple of 8-balls and have a fun couple of days without worrying about what happened to the apartment in the end.

I have seen a Meth head steal something from someone, and then politely offer to help them look for what they just stole from them.