I love warlocks, the class has so many potential roleplaying hooks built into it.
There are multiple ways you could make this work.
Ask the player - "Why did the Warlock make the pact in the first place"? Was he tricked into it by the patron? If so, why did the patron want him in particular?
Is the patron sitting back on their dark throne, watching their warlock struggle and fight while rubbing their hands together and muttering "exactly as planned."?
Perhaps the patron has an enemy, one who is capable of granting the warlock their abilities in the patron's stead.
Perhaps the patron tricked the warlock into the pact because this prevents the warlock from gaining some ability or accomplishing some goal - just by the mere fact of being a warlock. They consider the warlock's actions against him a minor inconvenience at best - their grand scheme will not be derailed.
In my games, I assume that having a warlock is a net benefit for the patron, regardless of the warlocks specific actions. As long as the warlock survives and gains levels, the patron gains an even greater amount of power (or some other equivalent benefit). The warlock may be actively working against them, but unless they do something really epic, it's just not going to matter in the long run.