As Dale mentioned about context, the word opportunist has synonyms that are better suited for certain contexts such as "a gold digger", a woman who wants to marry a wealthy man.

Self-seeker one who purses only his or her interests ahead of the best interest of others.

The word self-seeker is used in more formal contexts. In day to day conversations people will call opportunists other names depending on the situation. 

An opportunist could be a backstabber because he or she might betray their "friends" to get ahead with their ploy.