People used to cook with lard but that got declared as bad and the cause of obesity. Once you remove lard you remove the taste, as a result we ended up substituting with sugar and such, and obesity problem skyrocketed in the west, especially the US.

Diet and nutrition are complicated and there's little universal consensus. I think there's broad support now for the idea that dietary cholesterol does not become blood cholesterol, though. Back in the 70s/80s, the USDA heavily pushed low-fat/high-carb diets. It's a direct, significant cause--along with other contributing lifestyle and food processing factors--of the obesity epidemic that really took off then. It still influences dietary decisions today.

I'm not a nutritionist, and there's still disagreement among professionals, but I think the ship has sailed on that thinking. It's clear that a high-carb, low-fat diet is bad for you, leads to higher blood cholesterol, leads to obesity and diabetes and a host of other problems.

As far as butter vs. margarine, their calories and fat content are comparable. Beyond that, the impact on health comes down to whether "trans fats" in margarine are worse for you than the fats in butter. Again, no one can agree, and the scientific evidence leads to different conclusions.

Like everything related to diet and nutrition, most people are talking out their ass, or are at best uninformed. Especially me.