So firstly to be fair, I get the sense that before Kagha started taking over the druids overall were fine with the tieflings. There are clearly some who weren't, but I get the sense that most of them were generally okay to shelter the tieflings. As for Aradin, it's not as though Zevlor straight up kept the gate closed. Forgive me if I'm misremembering but Aradin showed up in a panic, Zevlor asked where Halsin was (a fair question since he's in charge) and then when Zevlor realized what was happening he actually did yell for the gate to be opened. Then one of the tieflings in the middle of openning the gate died by goblin arrow so the gate shut again and focus shifted to fending off the goblins. Which was tactically justifiable because some goblins might have gotten in and Zevlor wasn't prepared to risk putting more of his people in the most vulnerable position possible in that scenario. You can also argue that if they had let Aradin and his people in, the goblins would have either fled to alert the rest of the camp, or just kept pushing in and actually broken through the gate, at which point not only would Aradin and his people still have had to fight them, but all those civilians would also now be at risk.
The tieflings may potentially be drawing trouble (which actually isn't true, the cult's main target was always the grove, you could argue that they weren't in meaningfully more danger with them. There were certainly issues arising but that's another matter) but letting the tieflings in didn't open the grove up to imminent danger right in the moment. Really Aradin's mad that Zevlor didn't open the gate fast enough, and when Zevlor did open the gate after the battle was over, Zevlor had very reasonable gripes with Aradin's rash choice.
