Father Grigori was once a pastor in the town of Ravenholm before the combine attacked using headcrabs. Very few made it out alive. The pastor survived the attack but was driven mad by the years of isolation. At first the 'mad monk' tried to remove the head crabs without killing the victims. After failing this he now destroys the infected with home-made traps and his Winchester rifle.
While in Ravenholm, you discover that Father Grigori is the only one still human (as far as you know) in the god-forsaken town. It becomes clear to you that he has obviously perfected the art of zombie killing. A first he speaks to you from roof tops, referring to the zombies as his 'congregation'. Later he gives you a shotgun and eventually helps you escape from Ravenholm.
The Father seems to have no remorse over killing the ghouls (as you escape from Ravenholm you can hear his uncontrolled laughter over the gunshots), but he does have a certain connection to them. He is set on releasing them from their hell. In his own words, "a shepherd must tend to his flock, especially when they have grown... unruly."
Note: Although Father Grigori is a friendly NPC, he is listed under the category of animal in Garry's Mod.