Our gaming hobby can be played at several different levels. You can play light-hearted pulpy fun, along the lines of your average Saturday morning cartoon, where the DM just has to make sure that the bad guys all either get arrested or they run away to be naughty again another day.
You have the middle ground, which D&D usually fits into, where it's okay for the heroes to kill an entire race of entities, such as orcs, simply because they're assumed to be 'evil.' However, the most evil thing they do is maybe raid or pillage a village from time to time, no worse than the Gauls. If you'd lived among the Gauls, you might not have thought them extremely charitable or nice, but they weren't so vicious and vile that they deserved to be cut down on sight. In many D&D games, IMHO, player characters already act with an impunity and self-righteousness far more evil than you will find in this upcoming book. Sure, it's heroic to fight evil, but in the average D&D game, orcs and their ilk aren't evil villains; they're simply monsters to be mowed through.
Finally, you have the type of games that the BoVD is intended for. Sure, the type of monster-slaying you see in normal D&D is cool and heroic (like something out of Beowulf and St. George and the Dragon), but it's somewhat trite heroism. In more serious games, the fight against evil is more emotionally-grounded. Villains are developed not simply by them showing up and twirling their mustaches (no offense, Sagiro
), but by having motivations that aren't clear-cut. In a game where even the bad guys might be redeemed, it takes quite a bit to convince the heroes to slay with impunity.
Let me use my own game as an example. First, let me say that I may have mismatched my plot to the playing style of the group, sadly. The PCs are in the middle of a war that involves a large human empire, a smaller human kingdom, two rival Elvish nations, and a large confederacy of Orcish tribes. Out of all the PCs, at least one is either a member of one of those groups, or allied with them. Before the war started up, there were some tensions, but no one in the party could look at any one group and say, "They're evil; let's kill 'em."
And now, the various groups have gone to war over the things that people go to war over, and the PCs are left in a rut, not willing to ally with any single side, since they don't want to fight any of them.
So who's the villain out of this? The evil bastard who plans to clean up after the smoke clears, of course. He set up events and arranged assassinations and hoaxes to make sure everyone would begin killing each other. After the fighting ends, he plans to take over himself. This villain is evil, in my opinion, and to make sure the PCs realize this too (so they'll want to stop him), I would not shy away from using something from the BoVD in one of the villain's plans. So far he's been using a neutral force of priests to 'bless' the bodies of the dead from all sides, so that on a given day, all of the bodies will arise as undead.
If the PCs defeat him, then they'll be heroes, not simply because they stopped the bad thing from hurting innocents, but because they themselves weren't swept up into the violence like everyone else. And defeating a sentient, calculating evil is always more gratifying that beating an unthinking brute of a foe.
That is why the Book of Vile Darkness will be finding a place on my shelf.
