Why would the mafia want to force Frank to continue to work for them (one reason I I think is that he's a 'wizard' but I bet there's more to it than just that) esp. when Frank had made it clear at the beginning that he won't do more than 1-2 jobs.
The shopping center investment seems like a move to turn black money into white money, though.
Sounded like a way to scam him out of a good chunk of his money.
Once the mob owns you they own you from what I've read. He'd must have been dumb to get involved with them in the first place.
And even though the end seems to wrap things up, they wouldn't be. He'd have to keep running because they'd be after him...I mean the thousands of others he didn't kill.
I think Frank made an emotional decision. When Jessie popped into his life, it drove him to accelerate his tempo of earnings and against his better judgement, hop in bed with the devil.
Funny thing is, although he is a bit spendy, it still seems like he was earning close to a million a year from the scores he took down. He could easily have a huge nest egg stashed away (once the launderers took their cut), and earned compound interest on CDs (rates to depositors were 5-6% around then, and shot up to double digits about two years after Thief was filmed).
But then again, a guy with a stash of clean cash opening up envelopes with a safely earned check doesn't make for high drama...
"...and Mrs. Taylor sure seems to use a lotta ice, whenever he's away."