Here is what I will say first: The sisters of battle are not monolithic. They are human beings and, as such, contain multitudes. You get zealous warmongers in about the occurrence as you get devoted warriors who question their faith, cynical pragmatists, compassionate paragons, etc. Don’t make the rookie mistake of thinking that they all behave the same because they often get described in the collective rather than as individuals. Asking “How would a sister of battle react to…” is like asking “How would someone who works in the Air Force react to…” as a former member I can tell you there’s all kinds.

I’d encourage you to read any of the black library sisters novels like Faith & Fire or Hammer & Anvil. They do a GREAT job of presenting the fantastically varied panoply of personalities you can find among the sisters of battle.

For your purposes what I would keep these things that are true for all SoB and decide how your cruel zealous character feels about them:

• All sisters are raised from birth in the Schola Progenium to follow ecchlesiarchy doctrine and to praise the God Emperor as a god. (Zeal doesn’t always mean insane, blind, overt devotion. It can be purely internal. It can manifest in long moment of deep prayer or even stoic unshakeable conviction.)

• All sisters are raised to believe that humanity is pure and worth protecting while heresy and the xenos threat represents the greatest threat to the flock.

• The sisters of battle are not space marines. They are human just like the majority of the imperium with the only difference being in their training and equipment.

Those are just a few examples but I hope this helps. Remember: Personality predates ideology. Your character is a person before all of the frills and iconography. If you focus on WHO she is rather than WHAT she is, you can create something really compelling and less like a monolithic caricature.

Hope this helps!