Criminals are just specific types of people, and people are just specific types of mammals. Being more specific sometimes aids understanding.

I think I get what you're saying here; it's an example of the non-central fallacy[1]. Calling someone "a criminal" calls to mind a set of stereotypes, to which blackmail/extortion don't quite fit (most crimes for which one gets called "a criminal" as a generic term are violent, for just one thing.) Calling them "a blackmailer" or "an extortionist" calls to mind a more accurate set of stereotypes, clustered more closely with how you'd react to kidnappers, con-artists, etc. than how you'd react to, say, a mugger.

[1] http://lesswrong.com/lw/e95/the_noncentral_fallacy_the_worst...