Saturated fat molecules have no double-bonded carbons, so they are long and straight, which means they stack easier and tend to form solids at room temperature, and solids are better at forming plaques in your arteries and interacting with cholesterol in the bloodstream.
Moving onward to hydrogenation, to my current understanding, if you hydrogenate vegetable oil all the way, you get saturated fats (no double bonds anywhere because you've shoved in so much hydrogen to occupy all the bonds). But for the purposes of making trans fats, it's usually partial hydrogenation.
I assume that most vegetable oils that get partially hydrogenated must be polyunsaturated fats to begin with (meaning more than one double-bond) because partial means you aren't replacing all the double bonds, so there must be at least two double-bonds present.
The result of partial hydrogenation means you remove these double bonds and start straightening out the chains, which makes them easier to stack (which is why trans fats tend to be solid or semisolid), like saturated fats.
Why are trans fats worse than saturated fats? It looks as though partial hydrogenation is simply taking a polyunsaturated fat and bringing it closer and closer to the status of a saturated fat, but not all the way. If the goal is to get a fat that is solid and lasts longer, why not just use a saturated fat?
And for whatever reason saturated fats aren't the answer: Isn't the end result of partial hydrogenation technically just another unsaturated fat? I thought unsaturated fats were considered healthy? So how are trans fats worse than fully-stackable saturated fats?
Or upon reflection maybe my question is, why are trans fats considered so much worse than other unsaturated fats? If all hydrogenation does is break up double bonds and insert hydrogen, straightening out any cis-kinks in the chain, doesn't this just generate the same acid with fewer double bonds and straighter chains? How is this any different than going out in nature and finding the same unsaturated fat, rather than going to the trouble of converting?
What's causing the difference, here? Where is my understanding breaking down?