The first line and the interaction with player after the failed attempt to kill us in sleep tell us, in a obvious way, that she clearly does not like betraying others.
It doesn't come as easy thing for her. If she has done it before, which very likely was the case, as she is a drow; it wasn't out of her choice, or there was huge feeling of guilt left afterwards, combined with being really conflicted over what to do.
Then we got the fact that she dislikes Lolth, a goddess of betrayal, backstabbing and cruelty. Another hint.
Yeah, no one wants to kill people that they like, she has no problems with killing innocent people that she doesn't know personally though, there is zero hesitation or remorse on her part when it comes to slaughtering everyone in the grove. She hesitates killing the player because she likes the player, personally, it's like if you were a servant of the absolute and absolute asked you to smash your nice car that you like. You belong to her, remember?
That is not really suprising though, if you judge her with consideration of where she comes from. Article on wiki from the link above might let you understand about it more.
But so does she belong to you. Do not forget, that would her deception towards the Absolute be discovered, things would go really badly. She risks her life and position just so that you are alive and even provides considerable help, just so that you stay safe. This is already a heavy improvement over typical drow behavior.
That's why I wrote above, about her being multi layered character. You will find both bad and good traits in her; There is potential to turn her towards one of those sides, that's for sure.
As for the innocent characters, noone can deny that she isn't willing to attack the grove. But what I found really interesting, is that she de-humanizes the inhabitants of grove by calling them "infidels" and "filth". It's probably her trying to rationalize the entire attack; we do know that she is sadistic too. This coupled with thirst for power and she sees it at that moment as perfectly fine.
This doesn't mean that she won't regret it later though. Take look at Nathyrra from NWN, they share some simliarities.
Thing is, we've yet to wait for the game to show us the true reason, for the attack. It's probabably because she wants to impress her superiors, but there were some interesting possibilities with
Zevlor being revealed as a spy for the Absolute , through datamining. It could be personal rivarly for the favour of the Absolute. We do not know what is the real future fate of Tieflings too.
Either way, it's really stupid that we have to kill Minthara or Halsin. They are really interesting characters and there is little reason for both of them to die.
Even for evil playthroughs, there is reason to have both of them stay alive.
