The captain of the guard is a fighter/paladin of Heironeous. He's interested in justice/restitution, not punishment/revenge.
A couple of prominent citizens could vouch for them, but at this time that would amount to political suicide. A few of the people they webbed would have stood behind them had not they been "targetted" as well. The most troubling thing for me to reconcile is punishing them for their wanton disregard for the safety of everyone else at the ball. The PCs just went in with guns blazing. How to punish them suitably without ending the campaign?
From a metagaming perspective I don't want to just slap them all in jail and let them escape. Our group has been faced with imprisonment in 3 other games over the past 2 years (two modern settings and one fantasy setting), all of them unwarranted uses of railroading -- heck, two DMs used imprisonment as a plot device to get their campaigns going. On the other hand, the PCs' actions here clearly warrant imprisonment.
Putting them in the stocks and being publicly flogged would be great, then forcing them to pay restitution to raise the people they killed. But the player's would never go for the flogging.
And like I said, the guy they attacked is evil to the bottom of his black shrivelled heart. They just don't have concrete proof of what he's up to -- just a lot of circumstantial evidence. Any cult attack on the PCs now is going to make them look like they were right all along. And there's a cult plot afoot to make it look like the city lord dies during the night -- so he can continue his plan to bring about the end of the world at the next full moon, two weeks a way -- while the PCs are under heavy guard. If they escape from captivity, they would certainly take the rap for his subsequent "death."