To me, this is the whole point of being a warlock instead of a cleric.
A cleric's relationship with their deity is one of mutual cooperation. You and your deity are mostly in agreement about how things ought to be. The deity gives you power and you use it in service of those goals.
A warlock's relationship with their patron, however, is at best a wary truce and can easily be downright antagonistic. That's why your power is a pact -- a bargain, a contract, with specific terms binding on both parties. So long as you don't breach the terms of the contract, the patron must provide you power. If the contract does not specifically forbid you to work against the patron's interests, you are free to do so. Even if it does, you can look for a loophole.
How else can you possibly make patrons like the Fiend work? If the Fiend can withdraw the warlock's powers at will, then the warlock is nothing but an instrument of evil. Anything you do is with your patron's permission and approval, and therefore serves an evil cause. There's no way for them to work within a good-aligned party.