Halsin's reason to go to the temple, was to find out why all these tadpoled creatures appeared from everywhere and converged to that place. (Nettie's words)
So he had to find a way to infiltrate the temple and get that information. Joining Arradin was probably a way to do that. (Let the adventurers attrack the attention of the guards while he could sneak in)
Once inside and captured he would find out who the leaders were and then probably decided they needed to be eliminated ASAP.
