DŌMO, I am SENSHIN.
Fujikido Kenji's NINJA Soul is that of NARAKU NINJA, a NINJA who wishes to slay NINJA. And thus, Fujikido becomes NINJA Slayer. I understand why Fujikido wishes to slay NINJA - his family was killed by the NINJA of the SŌKAIYA, so of course he would want to perform FUKUSHŪ1.
But what is NARAKU NINJA's motivation? What drives him - a NINJA himself - to slay other NINJA indiscriminately?
1 Note: FUKUSHŪ means REVENGE.