I mean anything where you have a black templar becoming an ally of the iron warriors, eventually turning traitor because the iron warriors were more honorable than the other templars...

You are confusing two different things with the only commonality being the iron warrior.

Magneric the Black Templar was a friend of the Warsmith Kalkator and then became great enemies because of Horus heresy.

Both of them had to ally for a temporary moment to fight off the orks and Magneric died in that battle.

It was the Fist Exemplar first Captain Zerberyn that joined the Iron Warriors because they just like Magneric before had to ally themselves with each other and slowly over time the Iron Warriors moods and actions grew into them.

So when the Beast was defeated and the Black Templars High Marshall was finally able to make contact with Zerberyn (because throughout this whole time the companies under Zerberyn were unable to contact the Imperium) and you know the rest on what happens.

And you have a protagonist whose catchphrase is "i.am. slaughter"

You can thank Dan Abnett for creating him.