I know it sounds harsh, but he made a promise to his father as he lay dying that he would take care of his sisters. His horrid wife talked him out of it. But where is his honor?

We know that he at least lived with them for some years as Marianne told us.

I agree that a good wife might have improved him. But he is pretty bad himself. Almost everything that comes out of his mouth while talking to Elinor in the London chapters is offensive.

I won't quote whole chapters to you though, but here are some highlights, or lowlights...

Ch. 33 When Elinor runs into John in Gray's shop, and they talk of Mrs Jennings and the Middletons as excellent neighbors.
"Excellent indeed. Their attention to our comfort, their friendliness in every particular, is more than I can express."

"I am extremely glad to hear it, upon my word; extremely glad indeed. But so it ought to be; they are people of large fortune, they are related to you, and every civility, and accommodation that can serve to make your situation pleasant, might be reasonably expected....
Elinor did feel a little ashamed of her brother;

The nerve of him to say such a thing. Is he not a nearer relation, who actually made a promise to help them?
I also got wondering, I don't think Elinor and Marianne knew of his promise to his father, since we only learn early on that Mrs. Dashwood knew. I doubt she would have told them.

He give her hints to "fix" Colonel Brandon, and hints of prior attachments that are impossible.

Someone mentioned about the 200 pounds given to Fanny by her mother like pocket change, as evidence of how she favored Fanny over Edward. But John's bringing it up to Elinor is in poor taste I think because it is more than her a third of her family's yearly income. And more than the yearly income that the girls would have had from the most generous gift he had considered giving them- if he had actually done it.

Now here is a sly insinuation from Elinor that is completely lost on him
"Certainly," said Elinor; "and assisted by her liberality, I hope you may yet live to be in easy circumstances."

Then he has the gall to suggest that Mrs. Jennings should leave her money when she dies to Elinor and Marianne.
Whereas, in my opinion, by her taking so much notice of you, and treating you in this kind of way, she has given you a sort of claim on her future consideration, which a conscientious woman would not disregard.

I'll quit here because this is getting long. This was only from Ch. 33.
Ooh he's a creep. I am quite put out!