They basically sold samples of 4 different strains of a gnarly zombie virus, the lab used to make them, and all the notes the crazed wizard used to make them. And 1 of the party members almost died because of one of the diseases on the way back to town because he was out of spell slots, the disease kept lowering his CON stat, and part of the sickness was preventing him from getting the benefits of a long rest.
In terms of bad decisions the party has made this is definitely up there AND ties into some of their other bad decisions quite nicely.
It's also going to solve a problem they've been ignoring by making it a different problem in terms of a near by area where a port city was conquered by a different crime lord who has declared themselves a new feudal lord and that that area is his.
The crime lord that they sold the zombie plagues to would rather see that port city in ruin than the hands of another, and he now has the tools to do so with as simple as a contaminated food shipment passing through the area.