If someone wants to start teaching, the most important quality is just that--they want to teach. Of course, they need students, but if they have only a single student, they are teaching. That's the only requirement.
To be a good teacher requires more. That requires knowledge, research, and experience. Being a good teacher requires the new teacher to try to become better, and to seek out different avenues for becoming better. It requires time.
In the end, teaching is a skill like any other. It has to be honed. And you have to start from somewhere.
In Deadlift Dynamite: How To Master The King of All Strength Exercises, Pavel and Andy Bolton remind the reader that "You'll start off with bad technique--everyone does." That applies to a lot of things, teaching included.
We forgive people for poor technique because they are new practitioners. Perhaps we should forgive new teachers for poor teaching, because they are new teachers. Everyone needs to start somewhere.
(Of course, like a lot of things, this has limits. It's forgivable to be an unskilled teacher when you've been teaching for a week. If you've been teaching for a decade, you should be better than when you started. There is also such a thing as negligent teaching.)