(CNN) If an adult male mocked a teenage girl, you would condemn it.

And rightly so. Because that is the sort of thing that -- no matter your politics -- we can all agree on. That adults should stay away from mocking kids. Especially publicly. Especially especially when more than 60 million people will see the mockery the instant you do it.

Which brings me to President Donald Trump. And a tweet he sent late Monday night about 16-year-old climate activist Greta Thunberg.

"She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!" Trump tweeted sarcastically above a link to Thunberg's speech at the United Nations General Assembly meeting on Monday. "People are suffering, people are dying, entire ecosystems are collapsing," Thunberg told the UN audience. "We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth."

Was she passionate? Yes. Was she upset? Yes. Does she have every right to those feelings -- particularly given the dire predictions about the warming climate and what it could mean for our planet? Absolutely yes.

Unfortunately, it's not at all surprising that Trump saw fit to make fun of Thunberg's passion and emotion. This is who he is -- a schoolyard bully who doesn't differentiate between a 16-year-old girl and Joe Biden. Or a Gold Star family. Or a prisoner of war. Or white nationalists and those protesting their ideology of hate.

Even Trump's former communications director-turned-recent public critic, Anthony Scaramucci, pointed out that the tweet was a low blow.

"Parents in America and around the world: he went after a 16 year old girl yesterday. ⁦@realDonaldTrump⁩ unfit to serve," Scaramucci tweeted.

What is truly depressing is that Trump's supporters will defend this attack. Here's how:

1) He wasn't joking! He was totally serious in his praise for her!

2) He was joking and it was freaking hilarious! Thunberg was yelling and shouting! She got what she deserved! All you liberal media snowflakes can't take a joke!

On the first justification, Trump's default approach to those with whom he disagrees is sarcasm. The idea that he posted a totally genuine tweet praising Thunberg as "very happy," when it's quite clear she was anything but, makes it obvious that Trump was mocking her. If you can't see that, it's probably best you stop reading right here.

On the second excuse, well, that's the real problem at work in the country today. If everything can be justified by how much it allegedly freaks out the liberals or the media, then you can justify anything Trump does. Heck, Trump shooting someone in the middle of 5th Avenue -- an actual "joke" he has made -- would be absolutely hilarious under this logic to the President's supporters. Look at the media freaking out. Ha ha ha ha ha!

So, yeah. You might think I am exaggerating -- and, I am, slightly, to make a point. But the justification of everything Trump does as totally fine as long as it makes liberals and the media (and the political establishment more broadly) uncomfortable is a very slippery slope. And one that Trump's supporters seem entirely comfortable sliding down all the way to oblivion.

Here's the thing: Even pieces like this one condemning Trump for his comments on a 16-year-old girl -- and I can't emphasize that fact strongly enough -- will be lumped into the broad category of "Trump Derangement Syndrome" by the President's supporters. Trump is messing with the media so bad! They don't know what to do! He must be doing something right!

He's not. Take the politics out of it. Think of your own teenager or a friend who has a teenager. Would you want the President of the United States mocking him or her solely because what they believe is different than what he believes, and they have the audacity to speak out about it?

You shouldn't. Because there are some things that are beyond politics. Things that are right and things that are wrong. This is one of them. Period.