Why is "The Last Leave" by O. Henry ironic?Could you include a topic sentence as well?

To me, this story is ironic because of the fact that Mr.
Behrman dies from pneumonia.  This is ironic because that is exactly what he is trying
to save Johnsy from.  She is dying of pneumonia and because she does not believe that
she will be able to live once that last leaf falls.

So
Behrman goes and paints a leaf on the wall.  It convinces her to live, but painting it
kills him because he went out in such bad weather to do
it.

So my topic sentence would be something like "The Last
Leaf is ironic because Mr. Behrman dies of pneumonia because he was trying to prevent
that same thing from happening to Johnsy."