May his soul and the souls of all the faithful departed rest in peace. Amen.
I think "May the Lord have mercy on his soul" is a more appropriate formulization of a prayer for a notorious public sinner who died outside the Catholic Church, as it expresses the charitable wish of the petitioner without disregarding the serious objective evils surrounding the departed's life and death.
It likewise obviates the possible scandal that would arise if others, seeing your prayer, were to infer that you were adhering to the following error, solemnly condemned by Pope Pius IX:
17. Good hope at least is to be entertained of the eternal salvation of all those who are not at all in the true Church of Christ. -- Encyclical "Quanto conficiamur," Aug. 10, 1863, etc.