If you are meaning mephistopheles, lord of the hells, then recall that Zariel is a point of pride for him - an angel and divine champion that he brought the fall to personally. He does not want her gone - he wants her right where she is, serving him and on display for others to see at the same time.
Thing about mephalis is he is shown to have fits of rage and seek to destroy things when he grows bored. He is well known for killing his allies and servants. Its possible, albeit unlikely, he saw her as a threat or just decided to toy with her and intervene for the fun of it. Devil's by their nature are entities of curiosity. He detects mindflayers experimenting with dopplers. He is curious. He toys with us ... "go shop around" "i will be there when it ends".
However, as you stated above, it could just be looking to closely to explain bad writing. We will need to see more of act 1 to know lol.
