1. "Screw 'em" is a way to say you don't care what someone thinks, even if what you're going to do or say hurts them in some way. It's pretty informal, and just a little bit vulgar (I wouldn't say it in front of my grandmother, but I would say it in front of my mother). Maybe it's better to give another example: "My friends were teasing me about how I'm not good enough at sports. But screw 'em, I'm going to join the team anyway!" This is like saying, "I don't care if they think I'm not good at sports, or if joining the team means they don't win as many games, because I want to play, so I'm going to join no matter what they say." 2. Often a different thing is used to denote a point system, which is related to what you're rating it for. The most frequently used things are stars and thumbs up (I rate the movie 4 stars; I give this book 5 thumbs up). But sometimes, especially if you're rating something for a very specific trait (like how scary it is) you might choose a different object. Here, skulls mean scary in the same way that stars mean good.

I think that "screw" is a colloquial expression meaning something similar to "fuck". Screw ´em is the contraction for screw them. This leads me to believe that "screw them" = "fuck them". As for the skulls , he says "Shiver Scale". Something that frightens you, sends shivers down your spine so I think he wanted to award with skulls as they are associated with fear and horror films. I hope this helps you :)