Ah, the players who have realized no one can touch their characters and feel absolutely no restraint to their actions.
I don't believe you mentioned how old your players were, so first, I sympathize with your dilemma of wanting them to have fun but at the same time not run amok through your campaign world, and spoiling your planned plot line.
My friends and I have discussions similar, usually when we, as players, miss a critical clue from the gamemaster and therefore can't continue the plot as crafted. When I GM and this happens, I start considering how I can gently nudge the PCs back on course. It can take some time, and sometime some heavy-handed hints, but it generally works.
The first thing I always consider - the PCs don't operate in a vacuum. NPCs live and work in the campaign world, and both the "good" and "evil" sides have their own agendas and motivations. As the PCs are in motion, so are the NPCs. This doesn't mean I spend a lot of time self-narrating what the NPCs do in detail, but when the PCs do something, I spend sometimes 10 to 30 minutes considering how their actions affect the NPCs, and what they will do in response.
For example - your players razed a village. Depending on how remote the village was, regular travelers might come across the ashes. Those NPCs might wonder what happened and take it upon themselves to investigate. Speak with Dead is a 3rd level spell, unless the PCs managed to destroy all trace of every inhabitant, one of the dead villagers could point a finger (figuratvely) in their direction.
Second, villains are villains for a reason - they are out for themselves. The PCs are willing to work with the necromancer. What does the necromancer get out of it? Whatever the PCs get from working with him, he will always make sure he benefits more out of the deal. He now has a group of patsies he can divert blame for his actions onto. The PCs have been bequeathed the silver crowns from one of your previous characters - the necromancer can now separately throw out information connecting the PCs misdeeds with their original benefactor.
All this can end in serious social ramifications for the PCs. Smiths won't forge special equipment, mages might not agree to enchant a magical item, clerics won't provide succor or raise their dead characters. Even merchants might artificially raise their prices when dealing with the PCs. Even something so simple at the bartender spitting into their ale tankards.

Great thoughts, and I'm already on that sort of track myself, as there's actually been 2 sessions since I recorded the video.

The kids range in age from 13 to 15, and I've started playing up the Evil Characters aren't really team players aspect. As although they're doing their best to ingratiate themselves with the Necromancer, her other followers are working against the players as they see them taking away from their power with the necromancer.
For example, one of them has already sent Cultists against them to get rid of competition.
I've also had their activities for the Necromancer noticed, so they are being tracked by several good characters, so will be bringing trouble with them when they next meet with her.

The idea about having the dead villagers questioned using a Speak with Dead spell is excellent. I think that might be an excellent trial scene, having the dead speak against them when they're sure there's been no witnesses.

Glad I could help.
Currently trying to get the group I'm taking a turn a GMing away from the "20 minute workday." Spell up, engage in one fight, then retreat to a safe place for the rest of the day. They've retreated far enough away for 4 days to some allies who could help them avoid a scry spell. Well, the villain isn't liking that tactic, so he killed some lower level allies of a different faction while they were away. We'll see if that lights a fire.

Yeah I made sure a long time ago that my PCs knew their actions would have consequences in the world., and that murder hoboing as it would come to be called would bit them in the ass regularly and in style. One thief who kept stealing from the party to the point that they complained and he would just argue it was all in character died in a black pudding trap when he left the pcs to face 2 ogres so he could loot the pile of gold in the floor

another group of PCs found out economics of the campaign world when after successfully slaying an older dragon and securing its horde of treasure. they were shocked to discover that 5 lowly humans just didn't have the carrying capacity to haul THE METRIC FUCK TON of treasure that was piled everywhere and which included 10s of thousands of golds worth of gems coins statuary and artwork, magic items, chests of various items. they had to go get 6 carts worth and sacks and chests to stack it up and haul it all which was an epic endeavor in and of itself and THEN had to deal with tax collectors. villagers affected by the dragon villagers affected by a local disaster. monsters and various armies looking for the treasure to claim it for their respective kingdoms, a marauding party of adventurers who were looking to claim the treasure themselves without having to face a dragon for it and so on and so forth

by the time they were done they basically were so flustered that they dumped the majority of it into a nearby ocean except for a few choice bits and went back to adventuring