Fats (in and of themselves) aren't bad, it's everything added in with them. The traditional diet of the Inuit Indians was amazingly high in fat (whale and seal blubber), yet they had the lowest incidence of heart disease and cancer. We changed that, outlawing most of their whale and seal hunting, and substituted fast foods and the typical crap that grocery stores have been pushing since the '50s. Their incidence of heart disease and cancer rose until it leveled off in the '80s, about the same now as the rest of us Americans.
(btw, I'm an organic vegetarian, and I preferentially use peanut oil and sesame seed oil when I stir fry...)
Taking ANY oil up to 300+ degrees and holding it there causes the molecules to close off the oxygen bonding sites: it 'saturates' the oil, making it biologically as useful as tar. The long the oil is held at temperature, the more it saturates. French Fries at a fast food joint are particularly toxic, as the oils have been heated for hours before changing.
Trans fatty acids are merely an extreme case of this. They were originally invented to get us away from the 'dangerous' cooking oils - palm kernel oil and coconut oil, implicated in lots of heart disease in the '70s and '80s. Oops. The cure was worse than the disease.