The more evil a man is, the more attractive he is. I heard about something like this years ago. But what do you think? Yes, ヒール[hiiru] means heel in English; a man behaves badly toward other people. So old-fashioned way, though…系[kei] means a type of person here. So ヒール系 means a man who should be classified as a heel. I can think of Light Yagami(Death Note), Azuma Yunoki(Kiniro no Corda-primo passo-), or Keigo Atobe(Prince of Tennis) for this kind of person. Anyway, such a ヒール系 is becoming very hot in Japan now whereas they are so evil.
Having said that, in my opinion, Japanese ヒール is slightly different from English heel because, for example, Yagami Light is surely evil, but he behaves so nicely in front of others. So I think Japanese ヒール系 should mean men who are so evil inside whereas they are so nice in front of others.