Is it OK to submit a paper without asking your former supervisor? It has been nearly four months since I finished my Master's degree and I do not yet have any publications.

Around this time last year, I and my two thesis advisors submitted a paper to a conference and it was rejected with a score of "borderline paper" (unlucky me, that year there were so many quality studies).

Now, I look back and see that that paper was poorly written and did not have a clear message.

I am not claiming that I am the paper-writing-guru, but I believe that I have improved.

What I think is, without asking to my advisor, keeping him busy too much, should I write a paper and submit it to a conference? (of course, not a bogus conference).
Or should I be more patient and wait for him to approve a manuscript to submit it somewhere?

If I write a paper based on my thesis and submit it to a conference, is that accepted as doing something behind someone's back?

Edit: I see that I should not submit a paper based on my thesis without co-authorship of my supervisor. What about submitting a paper based on a different topic within the same area of research?
You should never submit a paper without the active consent of the co-authors.  If your advisor is a co-author (and they probably will be for work based on your Masters thesis), then you definitely must have their consent to submit.  You can do most of the writing yourself, if you want, since you probably have more time to devote to this than your advisor, but you should talk with your advisor about your intent early in the process and must determine authorship.

Addressing the follow-on question in your edit: new research conducted independently from any work done during your Masters, however, need not have your advisor as a co-author.