it depends on how much you spam the necro spells, basically there are two ways to build a sampire, wraith form with a little bit of spirit speak while using curse weapon or vampire form (the ss points instead all in necro) while using a mana leech weapon.
in the first example the curse weapon spamming will in the end lower your karma but you can even it out depending on what you are killing
in the second example you only cast necro form once and thus only lose karma once (until you have to recast it)
in either case its generally not too hard to keep your karma pretty high especially if you are killing strong monsters that give decent karma gains