There's another power in play (Lord Talbot's patron) that the group doesn't know about yet.
When Lord Talbot was confronted, he said "I was promised a kingdom!" It's unlikely that the castellan of a small kingdom would have access to kenku assassins or mind control magic. Furthermore, his corpse was taken from the morgue by the assassin; normally, an assassin has no reason to bother with a dead employer, unless they're that
loyal or they've been paid to do so (and what we saw of that assassin makes me highly doubt loyalty). All this points to someone with more power and resources who had an interest in helping Talbot. The obvious answer would seem to be the elves, but recall that even Esgalion and Tamier were surprised when the Witch Queen attacked King Donovan; there's no way they would have planned that out. That means that there's an unknown person or persons who would benefit from having Sarmanath in Talbot's hands.
- Your almost certainly right about there being a there being another group involved however I hasted to point out that Escalion and Tamier were probably not informed of the real nature of what they were doing. If only to avoid the various forms of detecting ill intent that D&D probably has.
Bayben has an imposter.
The reason as to why Bayben has a reputation for being both a hero and a butcherer is because there are in fact two men running around using the name. One is a brave and noble hero and the other is a violent religious extremist. Bayben is searching for this imposter and the party will eventually have to battle this fake Bayben.