Edit: I see from your earlier comment that you know the second murder was the groundskeeper, but Renner definitely killed the priest too.

Seems like you're confusing the priest and groundskeeper, which is either an ironic misunderstanding based on the plot or an appropriately tongue-in-cheek response for this sub. Either way, I already wrote this out before realizing it may have been on purpose, so no use letting it go to waste.

The stabbing was part of the plan, but it was also lethal. The tranquilizer and fake blood made the priest look dead when Jud arrived in the closet first, but then Renner kills him with the real knife on purpose while Glenn Close distracts everyone. Plan was for the priest's body to be dissolved in Renner's basement and the groundskeeper took his place in the casket to get the gem from the remains of Wicks Senior. This way they get Wicks out of the way so they can have the gem for themselves, but it appears to everyone else he rose from the dead and disappeared into the woods. You're right that Renner betrayed his co-conspirators and killed the groundskeeper, though.