You can only attack hostile creatures, with the exception of scripted sequences (ie shiving the prisoner). So, no, you can't just kill her on sight. You can't kill the King on sight, or Duncan, or another other NPC.
Now, you don't have to have her in your party, but that doesn't mean she stops playing an important role in the story.
If you mean party members, you're incorrect. There's one party member you have to take on, and you don't have to keep him.