The saturated fat debate should have a been a non-starter, and probably would have been if people had the internet in the 50s, 60s and 70s when the science was done.
About a century ago, humans dramatically started changing their diet, notably with the introduction of refined sugar and vegetable oil (often processed into hydrogenated or trans fats). Ancel Keys, and the saturated fat researchers came up with the "lipid hypothesis", that fat sticks to the arteries and "clogs them up". They didn't even consider the new foods introduced when human health declined, but decided that it was something that we've always eaten which must be the problem. The reason people started suspecting cholesterol was because we'd just come up with ways of measuring it in the blood - so they took the data and went looking for "problems". It really didn't make any sense.
To quote Nutrition and Physical Degeneration, "Statistics have been published by the Department of Public Health in New York City which show the increase in the incidence of heart disease to have progressed steadily during the years from 1907 to 1936. The figures provided in their report reveal an increase from 203.7 deaths per 100,000 in 1907 to 327.2 per 100,000 in 1936. This constitutes an increase of 60 per cent. Cancer increased 90 per cent from 1907 to 1936." This is where things really started going south for humans, and cancer, arthritis, alzhiemer's, heart disease and diabetes really started to come into the picture. We have managed a continued increase in degenerative disease over the last 70 odd years since then. Today 1 in 2 persons who live to old age will die of cancer. Humans used to be able to live to that same age and have a 1 in 1000 chance of dieing of cancer.
Saturated fat is present in ever increasing quantities the closer you approach the equator. It's better suited to plants in warmer climates, as you move to the poles, polyunsaturated fat becomes more present since it has a lower melting point. Humans evolved in temperate regions, where saturated fat is more present. There are a number of studies done on natives eating high-saturated fat diet who were disease free (The Masai for example).
Today we have hypotheses (based on information we've learned since the "lipid hypothesis" about how fats work in the body) that PUFAs might be deterimental, since we know they go rancid easily. Over consumption of PUFAs in conjunction with an anti-oxidant poor diet and a diet low in saturated fat (combing saturated fat with PUFAs makes PUFAs dramatically more stable from rancidity), means that these fats can go rancid in the blood stream - when these happens these fats can no longer be used as fuel, and the immune system needs to clean them out. Many PUFAs (corn oil is the worst) are also higher in omega-3 and low in omega-6, humans have eaten extremely varied diets, but one constant is the ratio between omega-3 and omega-6, because of this constant, these fats are used as inter-cellular messengers for ramping up inflammation or turning inflammation off. Eat a diet of only omega-6 and no omega-3, and silent inflammation turns up in the body and becomes a constant drain on the system.
Still, I don't think that we will find any one fat sub-type as a true "enemy" (sat/mono/pufa - not counting fats destroyed by processing and unusable by humans for energy like hydrogenated and trans fats). All kinds of organisms use a mix of different fats, it doesn't make sense that animals would convert one type of fat to another in the liver, if that fat was harmful to them.