Is there a reason why leaving a covenant is considered betrayal? They are all working towards the same goal, to save the Shadowlands from the Jailer and keep the afterlife functional.
What should they care if I’m beating Mawrats with Necromancy, vampire abilities or fairy stuff? What’s important is that we are all beating Mawrats and rescuing souls from the Maw, weakening the Jailer and saving the afterlife.
Details are sparce because of Beta and my unwillingness to look into spoilers, but consider how bad things have gotten in the Shadowlands right now. Souls are disappearing forever. The lack of anima is causing all kinds of havoc. For the Night Fae as one particilar example, their queen is having to choose what “lives” and what “dies” due to the lack thereof. Leaving your covenant to assist another in their acquisition of the same could be quite easily characterised as a betrayal in this light.
They’re on the same side technically, but the covenants are very independent places. They keep to themselves and their own purposes. No one but trusted members are allowed in their inner sanctums. With the anima drought they’re all desperate to preserve their own side first.
You promised to fight on their behalf as BFFs, earned their trust, knowledge and power, then abandoned it. Which shows your word means nothing so they aren’t going to forget it easily.
You are definitely not failing them, and it’s not deserting if you’re leaving them to fight with their allies against the same enemy.
All covenants are in it together against the jailer, and an unrelated party picking a side to fight with then changing their mind can hardly be constructed as betrayal, unless the covenants are extremely petty.