I don't know if the DM that made this thread is still reading or not, but in answer to his original dilemma, there are a variety of solutions. One main issue is acquiring the scroll in the first place. Ravenloft is typically a very low-magic setting (especially compared to Forgotten Realms) and a Shapechange scroll should be extremely difficult to come by. If the domain is in Darkon, Azalin is the only person there that could make such an item (since he kills any wizard in his domain that comes close to rivaling his power) and I don't think he allow such an item to just be sold in the market. If it was any other domain, most Darklords would know, or quickly learn, of such an item and likely try to get it for themselves.
Involving the Shapechange process itself, I'd have the PC make a Horror check, or even Madness check, since he is becoming an undead creature and not just some other living monster. That might even be worth a Powers Check. And murdering a family in their sleep is an Act of Ultimate Darkness, automatically failing a Powers Check for PCs. This should put the PC on the quick road to becoming a Darklord.
If he is successful at creating enough shadows to wipe out half or all of the village, that should be worthy enough to create his own little pocket domain. What was the PCs plan after creating all those shadows? Maybe when the PC tries to leave the village with his shadows, he discovers that the Mists have risen up around the village and trapped him there, with his spell ready to wear off any moment, leaving him the sole living being trapped in a village of shadows desperate for the warmth of his living body.
And if the spell ended prematurely, he shouldn't normally be allowed to control the shadows still. The control was part of the Create Spawn power. And to have that power he had to be a shadow. He would be more than just a "killer", he would be a "killer (that was a shadow when he killed)". If he is no longer a shadow, it would be no different than the controlling shadow being killed, and thus leaving the spawn as free-willed undead. A regular "killer" cannot create spawn on his own, he would have to be a shadow "killer" to do so, and thus would need to remain a shadow "killer" to retain that control. Anything else is just arguing over the grammar. Anyone familiar with D&D can understand the intention of the definition of Create Spawn, and I imagine that includes everyone on this board, but a lot of people here just wanted to debate grammar and waste 3 pages of posting by arguing over wording.
The original Van Richten's Guide to Vampires, had some really good ideas about spawn. One was that the spawn obeyed the creatore because they didn't know any better. There was more info on that, which I'm probably real rusty on, but its worth looking into.
If none of these are sufficient consequences of the PCs actions, then consider that the Darklord of the domain would definitely become involved if an entire village is wiped out. Villages represent income for the more financially inclined darklords, while the more vile ones would treat them as a source of raw material. And having that taken from them by some upstart adventurer wouldn't be long tolerated. Aside from the fact there are not enough clerics and paladins to mobilize against the PCs actions, such a force would likely be beaten to the task by the Darklord or agents of the Darklord.