According to her own religion she is well within her rights to do exactly what she does,
According to their religion, gnolls are well within their rights to murder and eat literally anyone they come across. I doubt many people in the Realms would consider them to be justified.
Gnolls are demonic spawns of Yeenoghu, they would face the same attitude from druids as tieflings.
However I agree, most of the realms would not consider them justified which is pretty much the reason why they have their own communities far from civilization. That being said, as druids of Sylvanus within their own domain they have the right to do as they please as long as Sylvanus is not offended. Why? Because it's their turf.