Generally he uses really flawed ones, just to show what kind of G-d he is.
G-d uses imperfect people to show His perfection and do His work for His glory. Why? To show His grace toward people.
Now maybe thes person has truly repented and turned his life. And this is the Lord’s way of blessing this person.
I saw the author of the book “The Girl in the Green Sweater” (Krystyna Chiger) talking about her family hiding in the sewers of Lvov Poland during the Holocaust.
The man who was helping them hide was a petty thief (Leopold Socha) and they were paying him with jewelry the family owned. When the time came for the Chiger family to escape, Leopold gave the jewelry back because the family needed it more.