Zevlor says when you first meet him that monsters have been appearing around the grove and the druids blame the tieflings for it. So the grove was in danger. Halsin even knew that because he'd already had to kill one tadpoled drow. Halsin didn't consider the refugees a problem, but he took them in and then left the grove, meaning they had no advocates outside of Rath and Nettie, who couldn't sway the rest of the druids who seemed to hate the tieflings. When Aradin said "we're looking for the Nightsong" Halsin said "cool lemme abandon my grove and come with you" and lo and behold the random adventurers Halsin decided to team up with left him for dead at the first sign of trouble.
Nothing of this makes this statement:
Nah Halsin is buff but his story is just as dumb.
His story isn't dumb. 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.