I think in his mental thoughts on leaving, his oath should be a consideration.
The thing to keep in mind, Talarius behavior and that of many people in the 2 Churches is the result of the negative influence of Excrathadorus Mortis. That’s a big part of the darkness that Tiernon and Torean are trying to discover.
There are other agents influencing stuff as well, and thinking on this, a lot of what was going on with Sentir Fallon was the influence of Excrathadorus Mortis. He was deeply affected by it, and that explains a lot of the things he’s done/permitted, after killing Orcus.
The decision to work with Aodh & Lillith etc was a different story since that was before the dagger.