Home invasions/armed robberies on the owners of cryptocurrency exchangers. Not in the US though due to harsh prison sentences dished out for home invasions and armed robbery. Preferably in countries like Norway or New Zealand where one would only be looking at a few years in prison, nothing crazy.

The smaller exchangers hold minimum $10 million dollars in cryptocurrency and don't have any serious security. A couple of guys doing home invasions every few months worldwide and forcing the owners to transfer all the crypto at gun point would yield tens of millions of dollars annually. Do it worldwide and law enforcement will have a hard time trying to profile your next move.

Lets say I did three home invasions a year, that's $30 million dollars divided by two (because their will need to be two hold up guys). I'll be kicking up around $3 million dollars per year (20%). I think the bosses will be very happy with me:)