Have you ever seen anyone arguing against mental health help?

Yes, several times. Even this meme implies that arguing for more and better mental health services as a solution to massacres is foolishly wrong. Also, another reply I got here says:

Nah, we don’t very much need to worry about the murderous intentions, as long as they’re not able to put them into action.

You also can’t make mental health illegal overnight. People are born with mental health issues, it’s not something they buy at the store or grab from their fathers closet.

I think you are a bit confused about what I'm suggesting here, or I'm not understanding what you mean with this.

Ban guns, ban guns now. Fuck gun culture and fuck all gun owners (even the responsible ones)
We can thoroughly take care of the more complicated part of the problem once the easy part has been solved

You think banning guns is the easy part? History has shown us time and time again that prohibitions don't work. Even if possession of a single firearm was punished with death people would still own and trade them as it happens with drugs in places where its punished with death.
Gun control or even prohibition is like a small umbrella under heavy rain, you dont get drenched but you still get wet. We need a raincoat, a hat and rubber boots.
To be fair, better metal health services is not an absolute solution either, there are plenty more stuff we should improve in order to achieve a real solution.