'TMZ Investigates: Luigi Mangione: The Mind of a Killer' Now on YouTube

Luigi Mangione, the charged killer in the Brian Thompson murder case, enjoyed a privileged life ... but folks who know him say he also had a dark side.

We go inside Luigi's mind in a TMZ documentary that is now officially up on YouTube ... and if you haven't seen it yet, now's your chance to catch up on some shocking details.

Sources close to Luigi told us how Mangione became radicalized before quietly plotting to kill a healthcare CEO ... allegedly turning into an assassin despite a charismatic personality.

What becomes clear in our TMZ Investigates doc ... healthcare was not Luigi's gripe ... he was more concerned with corporate greed ... and he viewed the American healthcare system as a prime example.

We talked to folks who traveled extensively with Luigi, and uncover how he once turned down a day at the beach to hit up a gun range in Thailand ... plus we find out what book he tried to buy 400 copies of.