I prefer Jason to be grounded in reality. Most supernatural horror films I don't find remotely scary because I can't suspend my disbelief that Pzuzu has posessed a little girl and is making her projectile vomit green pea soup. Some supernatural horror movies I do find frightening, because I do believe in ghosts, so Poltergeist and other haunted house movies are frightening to me. And A Nightmare On Elm Street is based on an actual case of Sudden Unexplained Nocturnal Death Syndrome (or SUNDS, now called Sleep Paralysis). So that combined with how s**t your pants scary Freddy is makes that a particularly frightening movie. Aside from that, any movie about demons, devils, zombies, vampires, werewolves, etc just don't do it for me. They might be very well done movies, like The Exorcist, and I can appreciate them on that level, but they're not scary.
Movies about psychotic killers though? Even if they're unbelievably superhuman in their ability to take a beating and keep on coming, or display feats of superhuman strength, the fact that there truly are psychotic killers in the world today makes characters like Jason, Michael Myers, and Leatherface that much scarier.
As for how Jason could have all three genetic mutations? What if Elias Voorhees had Myostatin-Related Muscle Hypertrophy, and Pamela Voorhees had CIP? Somehow their combined DNA produces Jason, who not only shares their mutations but also develops Sclerosteosis. It would explain why he is capable of such feats as squeezing Rick's head until it pops like a zit, or survive being hanged and taking an ax to the head.
Maybe Jason just has MRMH & Sclerosteosis, and his ability to endure pain is just because he's just plain badass.
