Who says watching murder on TV is never sinful? If watching murder on TV makes you feel that murder (or violence) might be an enjoyable thing, or an OK thing to do, or worse still makes you want to go out and murder (or do violence against) someone yourself, then of course it’s sinful for you to watch it.
See that’s the important difference - porn is designed for one thing and one thing only, to arouse you - to make you want to do whatever sinful act is depicted in that movie. And people who it doesn’t do these things for won’t watch it for other reasons either, usually, they find it boring and incomprehensible.
Depictions of murder aren’t designed to make you WANT to kill or be violent to others. Although unfortunately they may do so sometimes, and for those people for whom they do it would be sinful to watch them, in general they don’t and aren’t.
Remember too that porn is real people having real sex, really sinning in different ways, be it fornication or whatever. By watching it you’re contributing (an accessory) to their sins. Murder depictions are NEVER real.