I don't think dismissal is fair, have an argument, calling it silly does no good.
Whether it's video games, rap music, films, or more recently, online content, the whole "x causes y" argument immediately becomes very black and white, with no acknowledgement of the massive middle ground that has to be traversed before you get from x to y. I'm going to use Pewdiepie as an example here, but I'm not at all well-versed enough in the specific nature of his content for this to be entirely accurate, so his name may be more of a placeholder than anything.
Giving him the benefit of the doubt, Pewdiepie is not a white supremacist, is not a Nazi, and is not in any way racist. However, he knowingly creates content, be it videos or just his tweets, that he must be acutely aware appeal to a sub-section of society that those tags can be applied to. Having a history in which he's paid people to hold up a sign saying "Death to all Jews," and calling someone a "n-er" while playing an online game, means he absolutely needs to distance himself from those tags, and the people to which they belong. He hasn't, and he continues to court them as an audience. He may not be racist, but at best he has become a prominent pawn for racists.
So, the vast majority of Pewdiepie's audience is more than likely made up of adolescent males, a group in which self-identifying as "edgy" is not at all uncommon. Pewdiepie's content seems to fit that bill, particularly with the growing prominence of meme-culture. With his audience not being a local audience, and for the most part existing online, his predominantly young audience will absolutely have members that are older, and more aware of the connotations of his content, and the context and weight behind some of the 'edgy' jokes he may make, and they will interact directly with the younger, perhaps more naive members.
So we have, for example, a 12-year-old boy who is an avid consumer of Pewdiepie's content. He sees the "Death to all Jews" prank, and sees the "n-er" video. He doesn't really understand why these things are taboo, but knows that they are. This kid may well then go on to make 'jokes' with his friends about Jewish people, and may well start using n-er or n-a with them, but it's all okay, because he's a kid, and he's joking. This kid doesn't jump from there, to being a fully blown white supremacist, and 99% of the kids in that situation get a few years older and look back in embarrassment, but not all of them do. Some of them end up on 4chan or 8chan, and from there find themselves exposed to the likes of Paul Joseph Watson. Through them they find a political group that holds ideas that they present as persecuted and marginalised, and that most importantly, present some other group as a direct threat to his way of living.
The process is not, watch Pewdiepie, then go on a killing spree. There's a longer process of radicalisation, like with all of these things, that gets ignored. It's far easier to portray it as one side blaming YouTube, and the other side going, "that's silly," than it is to recognise that the media we consume can shape our ideas and our behaviour, even a little bit, and that from that first step, there is the potential to just keep on travelling down a path that leads to hate-fuelled violence. 99% of people don't travel that path, and pointing out that a very tiny minority do, does not tarnish everyone who's ever done something remotely similar with that brush. However, it is important to recognise what the beginning of these radicalisation processes are, so that we can identify when people are starting to stray down dangerous paths.
You can't ban these 'gateways', because it's pointless, and the logical progression would be to ban pretty much anything and everything, just in case, which is insane. What you need to do though, is take a step back and think about why certain people identify with certain aspects of this media, on a level that goes a bit deeper than, "well, it's edgy."