Heating unsaturated fats causes them to break down and oxidize, leading to oxidized LDL, which is detrimental.

“when you heat unsaturated fats they break down they become oxidized peroxidation happens when you eat fats that are oxidized it's hard for your body to deal with these things it damages membranes you get more oxidized LDL this has been shown in multiple randomized controlled human trials with seed oils”