At the start of S03, Wendy is keen to expand, while risk-averse Marty isn't.
In particular, Wendy wants to buy the Knarlsons' casino. In S03E02, Marty initially manages (behind Wendy's back) to convince Mr Knarlson to not sell.
Marty (while spying on Wendy's phone) then hears that Wendy and Helen plan to go back with a much better offer. Marty then goes to Frank Cosgrove and arranges to have the Knarlsons' rival casino go up in flames, the idea being that without that rival, the Knarlsons would be less likely to sell.
Marty is almost always the cool, calm, rational, risk-averse, non-violent accountant. Why would he carry out such a risky plan? The effects of this plan were hardly certain--e.g. it might have messed up, the men could've been caught (as indeed they were on camera and then later shown by the FBI to Marty), it might've been too late if Knarlsons had already signed the papers, it might not have changed the Knarlsons' minds.
Moreover, if the deal did go through and the rival casino went up in flames, then yes, this would have been good for business at the newly purchased casino. But on the other hand, Marty and Wendy would have been under great suspicion (they buy a casino just when that casino's rival goes up in flames), which is exactly the sort of risk Marty was seeking to avoid in the first place.
(This could of course just be bad writing but I was wondering if there's any plausible in-universe explanation for Marty's decision here.)