He is incompetent leader, who even admits that himself. It's not a dumb story, it's normal realistic story about how people do make mistakes.
Yes ... Halsin certainly did a misstake ...
But i would not call him incompetent. :-/
- We dont know how ambicious Kagha was while he was around ... after all, his presence alone was more than enough to cool her down.
- The same we have no idea what creatures, or how often attacked the groove ... i mean, we know about litteraly two attacks of Goblin scout parties ... one is that Aradin lead there, and second one is that where was Sazza ... yes, we know that Drow with Goblins attacked Halsin and Nettie, but either i simply forgot, or there is never told where they attacked them ...
Was it another attack to Groove itself? Or they simply attacked them in wilderness, when they were i dunno ... gathering herbs prehaps?
Personaly i thrust the second option more, since:
1) The corpse is in the room, that is acessible from secret passage and locked.
2) No one else seems to know about the tadpole, nor the Drow.
3) Nettie is the only one who is able to unlock the secret passage.
4) Nettie gets that crown, wich unlocks it, from Halsin.
- And finaly, we have litteraly no idea how long Halsin was gone!
I mean ...
Halsin could easily leave in the time, where everything seems manageable ... and Kagha could as easily simply took and opourtunity of power vacuum, to iniciate a coup.
You say that druids hated tieflings ... until they had strong and respected leader who held a protective hand over tieflings, druids could be unhappy, but that is all (you can imagine it as angry people who swear when nobody listen) ... but once that protective hand dissapeared, and new leader even start to encourage the quarrels ... situation simply escalated.
And about his leaving ... i believe he had good reason. :-/ Goblins were attacking anyone and anything who, or what they meet in the woods ... there is Strength in numbers. And when Halsin find out that Aradin is goint the same way? Why would he not use such opourtunity?