I've always been a fan of MST3K and I've come up with a few of my own theories regarding the Mads.
I like to think that Dr. Forrester watches or had watched all these films beforehand, and as a result has gone mad. He wishes to spread the pain, and inflicts them upon the SOL crew. He works from the stand point that these films broke him, maybe they can break anyone, basically seeking to weaponize them. Forrester is abrasive and malicious, with no qualms about commiting violent acts. Mike/Joel's ability to take them in stride constantly irks him and I think drives him to continue his research in to worse and worse movies (and consequently propels him deeper into insanity).
Frank, on the other hand, has seen these movies and loves them. As a result Frank has not really gone mad, but just a little off kilter. He doesn't seem to hate the SOL crew or wish them too much harm, unlike Forrester. Even when he's being mean, it comes from his drive to keep Forrester happy (like Brent from The Big Lebowski, he's not really an evil henchman, but just doing a job). Frank seems to defend the movies as well, listing the actors he likes in them and not joining in on the lambasting of movies too harshly. Frank is like the opposite side of Joel, almost a "what if" Joel had gone along with Forresters evil plan willingly.