Manmukh is someone who, by following the vagaries of his mind, destroys his own precious chance of lifetime.
Saakat is someone who, by adopting trickery and deceit, destroys life of large number of other people too. The following verse by Dasam Patshah talks about Saakat..
ਭੇਖ ਦਿਖਾਇ ਜਗਤ ਕੋ ਲੋਗਨ ਕੋ ਬਸਿ ਕੀਨ ॥
ਅੰਤ ਕਾਲਿ ਕਾਤੀ ਕਟਿਓ ਬਾਸੁ ਨਰਕ ਮੋ ਲੀਨ ॥
Bhekh dikhaye jagat ko logan ko bas keen||
Ant kaal kaati katyo baas nark mo leen ||
Those who deceive others through their religious outlook and enslave common people, shall not only be subjected to cycle of birth-death (as opposed to liberation of a Gurmukh) but will also undergo reverse evolution.