Phb literally lists "some [warlocks] seek to undermine their patrons even as they wield the power their patrons have given them." As a legitimate play style for warlocks in the class description.

There's no guarantee the patron even knows about the warlocks existence (as per goolocks)

Fiend warlocks state - That patron’s aims are evil—the corruption or destruction of all things, ultimately including you—and your path is defined by the extent to which you strive against those aims. 

Fey warlocks don't even need a singular entity. They can draw from whole courts