The Hero's Path

Is "Empathy" prohibited on the map?

Unfortunately, skills cannot be prohibited. A Warlock needs "Guardian of Secrets" from education to use Empathy; perhaps the problem lies there?

P.S. Regarding Blackthorn, why kill her?

There are several options:
1) As a matter of principle, as a negative character.
2) If you want to take an alternative path to the dragon quest and earn about 30,000 more gold, albeit a bit later.
3) In pursuit of dragon artifacts, if you know that some of them are scattered among the enemy heroes.