Opportunism

Meaning - Pursuing one's own interests at the expense of others. 

It can also be called using people or taking advantage of people. 

This kind of behavior is not illegal, but it's unethical and violates basic decency and manners. 

Examples

Moving in on a recently widowed woman is a classic example of opportunism. 

Another example would be finding someone's wallet and not returning it. 

Taking money from a blind beggar's bowl is another such example.

Synonyms - Expedient, Time-Serving