base motive
Motivation for some action or actions that is guided by selfishness or moral depravity. It is clear now that his decision to marry into her wealthy family was led purely by base motives. I don't believe the defendant had any base motives in this case. I think she was well-intentioned but naïve about the repercussions of her actions.
ulterior motive, an
A hidden purpose, a secret reason for saying or doing something. The adjective ulterior originally meant “beyond” or “more distant,” a usage taken from the Latin and dating from the mid-1600s. Less than a century later it began to be used more specifically to mean “beyond what is said or evident.” It then was combined with several nouns—ulterior intentions, ulterior designs, ulterior purpose. Its frequent combination with motive is more recent.