the pervert after death

السٌَلامُ عَلَيْكُمْ وَ رَحْمَةُ الله

Can one make janaza salat and ask Allah's mercy on one who is known to be a fassiq?

Dear Brother, according to the Madhab of Ahl al Bayt it is not permissible to pray the Janazah for an unrepentant Faseeq and neither to ask for mercy for them.

al Imam al-Qasim Ibn Ibrahim al Rassi -alayhi salam- said:
It is not to be prayed on the Owner of a Kabirah (Fasiq)

And al-Shaykh Abu al Abbas rahimahullah said: Included in this is every Owner of a Kabirah, from the Rebellious and Highwayman.

(Excerpt from al-Tahrir, p.102, Kitab al Janaiz.)