I will say up front three things:
1. Maas does not want to join the Wardens. He only wants to join them to help defeat the Blight. After that, he will leave.
2. If this means you don't want him to see the Joining, simply tell him so when he arrives. He will leave willingly.
3. No, he will not try to kill Isala. Unless he has evidence that she has been lost to a demon, he will simply hate her entirely. For now.