May I reuse my own figure without coauthor's permission? I'm writing an editorial and would like to use a figure that I published with a coauthor a few years ago.

The copyright agreement (see #4) with the publisher allows the authors of the paper to reuse any of the work in any derivative work, as long as proper attribution is given, including a DOI.

Given the above, I would have permission from the publisher to include this figure in the editorial with proper attribution.

The coauthor and I, however, have had a falling out, and if I did ask permission he would almost certainly say no. He did not make the figure for the paper, I did, and I am first author.

Given that I produced the figure, and the publisher would allow me to use my work in any derivative work, am I required (even ethically) to ask permission of my coauthor to publish this figure in an editorial that I'm writing?

More generally, when copyright terms from the publisher are such that the author(s) may use their work in derivative works (with proper attribution), as is common in my field, must authors always obtain permission from all coauthors to republish orginal or derivative works?
You -- and your co-author -- have, most likely, assigned your copyright in the paper to the publisher. 

Without knowing all the details of your copyright agreement, your copyright agreement seems to allow you, and your co-author, the right to re-use the work, and that right applies to each of you separately. The right to re-use the work requires you to reference the original paper and all its authors. 

I see no reason why you shouldn't use the figure, even without the permission of your co-author, as your copyright agreement with the original journal appears to allow this. I see no ethical argument against re-using the figure, either. 

P.S. I have a qualification in intellectual property law, but I am not a lawyer.