The last two lines of Whitman’s Song of Myself, Song 22.

In his book, Leaves of Grass, Whitman writes in the last two lines in his 22nd song of Song of Myself

“What behaved well in the past or behaves well to-day is not such wonder;

The wonder is always and always how there can be a mean man or an infidel”

I’m having a little problem in comprehending those lines; did he mean that it is not important that a man behaved well in past and is behaving well today, what we need is that he should behave well always, i.e. even in the future?Or did he mean that there both types of people in the world: the well-behaving ones and mean ones?

Actually, in modern colloquial language “how there can be” is somewhat similar to a kind of exclamation for something unusual , for example

“How on earth there can be an animal like that?”

“How on earth there can be a corrupt man like that?”

I mean, to say it’s more like a question plus exclamation, but Whitman, I think, has used it for some assertion.