1. Obviously Tiamat and Bahamut are opposites
2. Bartholomew would like to know the head of the cult. Was it the half dragon he fought?
3. Bartholomew is now comfortable granting the wishes of these cultists by executing them. He has given them a fair trial, they have confessed to being the cultists that killed innocents, and they worship Tiamat.
4. His action will be justice not murder and therefore he feels justified.