Servitors are not made from corpses, they are lobotomized, modified humans who were “alive” when the procedure started.

Some absolutely wrecked humans (like soldiers who get partially blown apart) can be taken in for servitorization if an eager techpriest is closer than the hospital. It’s like a shittier version of how dreads are made. But more often it’s just prisoners and stuff.

I think there’s a line about this in one of the gaunts ghosts books, but it might have been someone punished