So I was going to try to catch everyone involved in chat, and then I realized the likelihood of that actually happening was somewhere between slim and none, so HERE WE GO.
The general idea is that a ring of low-ranking Cards working primarily in Deck finance - either at the Castles, or at the bank in Town - concoct a plan to siphon off funds from the Suits. Also involved - possibly even the mastermind behind it - is Nicholas, the Diamond psychiatrist. They start with trace amounts of money that shouldn't really be noticed, but as they get bolder and more overconfident, it comes to the attention of characters working in those departments - namely Zoe in the Spades and Rita and Iggy in the Hearts. NPCly, Leona probably notices in the Clubs and Lotty Berthier in the Diamonds.
This puts the pressure on the embezzlement ring, and especially Nicholas, who is keen to cover his tracks. He first kills Edison O'Brien, the Five of Spades who was their inside man for the Spades, and then Lotty Berthier, who was susceptible to his flirting but now suspects something.
These two deaths lead to Cadogan getting rid of this Problem Diamond by killing him and trying to clean up the mess.
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This is just what I've come up with after conversations with Song and Scy about it; obviously, other characters should be and can be involved! Particularly, I think it would be a great opportunity for the accountants to all get together and do stuff. And if anyone's like NO THIS WOULD SUCK CHANGE IT TO THIS about any of these things, I am totally open.