Well, there's certainly nothing stopping you from letting him keep his Turn Undead ability. When you design an NPC, you can do whatever you want with it, and the revenant is meant to be customizable to give it some more character than your regular zombies.
With that said, this is going to get weird. Here's why:
Each undead that can see
or hear you within 30 feet of you must make a Wisdom
saving throw. If the creature fails its saving throw, it is
turned for 1 minute or until it takes any damage.
From here on out, I'll refer to your NPC as you (like the ability does) to keep the grammar simple.
You are obviously within 30 feet of yourself, and you can definitely see and hear yourself. So unless you wear a blindfold and earplugs, you are going to affect yourself when you Turn Undead. So there's that. It's pretty weird, but unless you houserule that you can't turn yourself, it's going to get weirder, because:
A turned creature must spend its turns trying to move
as far away from you as it can, and it can’t willingly
move to a space within 30 feet of you.
So when you turn yourself, you have to spend all your time trying to run away from yourself. At this point, logic breaks down. You have to run away from yourself, but you can't move to a square within 30 feet of yourself. How this works out in actual play I have no idea. I'd guess that you either start running around like a headless chicken or you just stand still.
The nonsensical results this creates suggests that whoever wrote Turn Undead had no intention of any undead having this ability. Therefore, I strongly recommend, if you want to let this NPC keep Turn Undead, that you houserule that you can't affect yourself with Turn Undead.
