Had nothing to do with Shadow Word: Pain though. Horridon gives us permanent Twist of Fate uptime, so we do +15% more damage compared to everyone else. Combined with that we're one of the strongest execute specs, and the fight has us cycling through near-endless execute phases - and we Should be topping the meter on a fight like Horridon.
If the problem is Twist of Fate, the solution shouldn't be to nerf Shadow Word: Pain. Twist of Fate only benefits us on fights like Horridon, Shadow Word: Pain nerfs us everywhere.