Hum. I'm not so sure about the leaving people alive thing. Although it would be great if we could create less panic via not killing people, do you think that's realistic for peasants in a area that has had frequent killings by undead monsters to get attacked by a undead monster and then take the time to think "Hey! I didn't die! Maybe this one isn't worth calling in the legions of adventures for!" Stealthy, and that means killing, seems to be the way to go for now. Assuming we are evil that is.
Immediately, no. But in the long run, if everybody keeps swearing they've been assaulted by undead horrors of the night but nobody's died for a week, people are going to stop taking it seriously. Or at the very least, take it "this is obnoxious and scary" seriously rather than "this abomination is stalking us one by one, and it won't stop until nobody is left" seriously.
Conversely, if the disappearances continue, people are going to figure out that
something is (still) up.
Allies would be better. But
A: We have no idea if we can trust her
B: We have no idea if we will be able to collect enough life force for even ourselves at this point
C: We can just make our own totally subservient allies that have no chance to betray us, so why take the risk with the unknown?
A: Well no, but we should at least talk to her about it.
B: Certainly not, but if a pack of wights can feed themselves and their fat master, I'd think a pair could sustain themselves fairly easily.
C: Well for one thing, it's
really expensive. At least, for a minion that isn't made of tissue paper. For another, said slaves probably wouldn't be especially creative or motivated, being largely brute-forced puppets and all. Finally, we might want to get
away from the dickish wight thing, which is harder to do when you're murdering people to transform into your unwilling thralls.
Yeah, but killing your partner means they don't see it coming. Make of that what you will.
Prisoner's dilemma. Let's see if we can move towards the optimal solution!
Or at least make each other more useful than the life force they're carrying at this precise instant.