If you want to kill a dozen people and give zero shits if you die in the process (or even intend to kill yourself as part of it) what good is a ban?
Small words:
It makes it harder for you to carry out the thing you wanted to do.
An example:
A man wants to commit a mass murder. There are 4 rooms each with 12 people in.
In room 1 he only has his fists. He fails. The 12 people immobilise him.
In room 2 he has access to a knife. He kills 2 people before the other 10 disarm an immobilise him.
In room 3 he has access to a revolver with 6 bullets. He kills 6 people and is disarmed
In room 4 he has access to a semi-automatic rifle with a clip of 250 bullets. He kills all of the people in room 4, and the remaining people in rooms 3, 2 and 1 before he is killed by an armed response unit.
Do you see?
Better support for mental illnesses, restrictions on guns, places you can go if you feel overwhelmed, that is the way forward, a ban is futile.
I don't think that these things are exclusive.