Harshad's darkness stems rom this crazy obsession with Manik- he is so hell bend on destroying him, he has him down to the T except the Nandini factor. But why ? It cannot be coz of Alya or Mukti- because killing or destroying Manik has a very animalistic, barbaric and creepy feel to it.
He wants to inflict pain to Manik, destroy him and isolate- that is huge. the Why better be effective enough. On the contrary , Manik while aware of Harshad hasn't made it his mission to destroy him-so whatever happened in the past- moral victory was with Manik, and Harshad has resented it.