Undermining

If you willingly do something that you know will most likely greatly undermine your relationship with xyz, I'd have to say that you are at the point where you seriously consider your relationship with xyz rather screwed.

[for the times I've been an xyz, I humbly apologize to you]

But what if xyz values his relationship with you? What if what you do does undermine you relationship with him? Aren't you being rather selfish? Are you justified in taking this step?