Here's a Red John clue no one has mentioned yet from 'fire And Brimstone' (season 6)

Ray Haffner is on the phone with a client while walking into the diner to meet Lisbon.
We overhear him say, "listen to me Don, you don't have a mistress, you have a vindictive screwball that is about to ruin your life. [don replies] yes, well it's very lucky for you my job is to fix problems. so here's what you're gonna do: You are gonna write her a check-- It is gonna be a very big check-- And then you are gonna pray that this problem goes away. [don speaks]
If she doesn't, don't worry Don. There are plenty of ways to keep a woman quiet. OK? Ahright. I gotta go."

Clearly Ray Haffner has no problem killing a woman if she is a "problem".