And movies and video games cause violence too, right?

"Cause" is a vague (and impossible) way to describe this relationship, which is why I didn't use it. Someone could directly claim "I killed this cop because GTA5 made it seem like killing cops was cool" and it'd still be incredibly easy to find other more important causes than media consumption.

But, by and large, there isn't a recognized condition that makes people intrinsically desire to kill cops. There is, however, a recognized condition that makes certain people want to fuck kids (i.e, most of this subreddit), and at least some studies suggest that consumption of simulated/virtual CSAM makes it far more likely that the consumer will "move on" to real CSAM and eventual assault of children. Here's a link to a discussion, with sources and links, on the topic.

Weed is the gateway drug to meth and coke, right?

It's simply true that imbibing more available adulterants makes you significantly more likely to consume harder drugs than someone who does not. I don't think this justifies any of the policy decisions of the War on Drugs, but that doesn't change the fact that people who like to get high one way are going to be much more likely to try to get high in other ways (I say this as a drug enjoyer).