Generally, using magic against "monsters" is fine. Using magic against "people" isn't. People have souls, and ripping their soul from their body causes what in a different setting would be referred to as "a disturbance in the force". Monsters don't have souls and killing them causes no cosmic backlash. Note that people who behave in monstrous ways are still "people", as they still have a soul -- which may be twisted and blackened, but still can be redeemed.
There does appear to be some wiggle room in the definitions, and I think that this is largely an issue with magic being about who you are, and vice versa. That is, if you see someone/thing as a "person", then killing them with magic will make you more of the sort of person who would kill people with their magic -- even if the victim was, in fact, a soul-less creature. For example, I think Harry would have serious problems if he killed Thomas or Toot-Toot with his magic, despite the fact that both WCV and Fae are "monsters" with no soul. Your group should decide how to deal with this.