I have never heard of anyone getting a sign on bonus that was unconditional. When I have had signing bonuses they were owed back prorated if my employment ended for any reason in the first year.
Setting that aside, even if the work was boring, I would jump at the chance to earn $100M for several years of white collar, cushy work, purely for the impact I could have on the world with that money.
The guy immediately took leave to get some medical procedure done with a recovery time, then when he returned he quit for another job. He barely worked, collected a big signing bonus, used the company's insurance plan for a very expensive procedure, and then disappeared.
From that point forward, signing bonuses had the standard conditions attached.