Cause I haven't seen the video once. I haven't been offered to neither.
You have to take some responsibility at some point. Always asking the rest of the world to do good by you is only going to lead you so far.
Today it is you who says we gotta censor what you find offensive, tomorrow President Orange-Nimrod will censor all of CNN because it's OK now to censor things that somebody/anybody finds offensive, and in a few decades time President Jenna Bush Hager will censor all depictions of an Earth that isn't flat because those are offensive to the good god-fearing citizens who are true to the word of the Bible.
No one is saying the NZ video should be purged from the internet. But it is foolish to equate censorship with private social media companies scrubbing it from their sites. You are welcome to search out other venues that host the video, or host it yourself. Your rights are not being infringed upon if Twitter and Facebook won't.
Common carriers, like Telstra (largest mobile Australian ISP), started blocking websites hosting the video. That's not OK for an ISP to do. Same as it isn't OK for e.g. a utility company to cut off power to a "nazi" (whatever that means these days) or to Bernie Sanders supporters because the utility CEO doesn't like "socialism".
Private entities actually hosting the content would be a different matter, and the decision should be up to the private entity. Unless such a private entity is a de facto common carrier in itself. I'd argue Facebook, twitter, reddit, google e.g. are de facto common carriers running general purpose platforms - given their size, influence and reach - even according to their own PR. They are analogous to a utility company, and saying "you can host the content on your own server" is like saying "you can just run your own generator for your electricity needs/use solar panels and battery" or like saying "you can always create your own telephone network if you don't like what the companies are doing; all it takes is 2 old cans and a string".
In my opinion common carrier and de facto common carriers should not be allowed to outright censor lawful content. Videos of newsworthy terrorist attacks, no matter if it is made by a terrorist (e.g. this one) or just an innocent bystander (e.g. 9/11, Las Vegas), are not illegal and should not be allowed to be censored by such de facto common carriers/de facto utility companies.
Do you need everything to be labeled and rated by someone else before consumption?
Don't search for and click on videos of the shooting? Sure, it won't stop someone specifically trying to get you to watch the video. But censorship doesn't change that. No amount of censorship can keep people from, say, adding some noise to the video to circumvent any filters and sending it to you.
If you want to make it so that no site owner can moderate their own site, you are going to have a huge uphill battle. And I can't imagine why you would want that kind of world.