I see no option but to kill her in my playthroughs, because siding with the goblins is from my perspective a stupid decision - and not only because I like playing Tieflings (and I like druids) - I mean, you have a cult of mentally unstable and very violent people. On the other hand you have a group of innocent refugees and mostly kindhearted druids. I'm not counting snake lady, because everyone makes it very clear, that she will have nothing more to say, when Halsin is back - so one reason more, to save that chap, to bring back a bit of stability to this region. Why would I join the murder cult? Even as an evil character ... I mean, I'm evil, not stupid. Things like that always backfire.
I could see Minthara as an either/or character between her and Wyll, because I don't see Wyll agree to siding with the psycho murder cult. But in my case it will be Wyll all the time, if they don't drastically change the quest, to make it more believable to help the goblins (other than the 'I want the sex scene with the drow lady'-argument).
Right now, the character and the cult has nothing to offer me. There has to be major rewriting of the whole questline to be done - think of the decision in Dragon Age Inquisition to help the templars or the mages - that was a decision, where you sit down and really weigh your pros and cons for every side. I didn't even think of that in this questline. Of course I'll help the refugees and the druids, of course I save Volo and that goblin prisoner can rot for all I care. And no questions ask, I'll kill the tree cult leaders.
I know, that I'm writing more about the cult than about Minthara, but to me, she is just another crazy cult member at the moment. I don't see anything, that would screm 'Yes, this is interesting companion material' to me.