I’ve been thinking over this one for a LONG while now. But I’ve finally come up with a headcanon for Kakuzu’s obsession with money.
Often he claims that the only thing trust worth or dependable is money. Its the only thing that can really be counted on in this world. With that, given his background of being unfairly punished and scrutinized by his entire village for his selfless act can easily be used to conclude that the reason Kakuzu is so obsessed with money is because its a safety thing.
He knows for sure that people will do absolutely anything for cash. Its a necessary evil and it runs everyone. literally everyone. He is well aware that the more money you have, the safer you’ll be. The more powerful you’ll be. As long as you have cash to throw at people, or even take from people, people will do nearly anything you ask. It’s a safety thing.
Snatching up as much money as he can for so many decades has just pushed his obsession to unhealthy heights and now he can’t stop himself. It’s something he absolutely has to have in order to be comfortable with life itself since his trust towards anyone is so very limited. The only way trust is given is with money.