Margarine? That is nasty stuff and it doesn't belong in a low-carb diet. Full of chemicals and trans-fats. You are far better off with real butter. Or olive oil. I use olive oil for a butter substitute sometimes. Olive oil is very good for you.
I do use a lot of olive oil. But for chicken or for sautéing onions you just gotta add a little butter or butter surrogate for the taste. Plus for those few times we buy bread. (Yes, we do eat bread sometimes!) I forget: Does butter have better fats than margarine? They're both solid at room temperature, so I assumed they both had lots of saturated fats.