Any good WYSIWYG based latex editor software which might be useful for making instructional videos, say, in mathematics or other subjects?
There a comprehensive revision of LaTeX Editors/IDEs in this site with a somewhat standardized checklist of features, and one of them is the "Built-in Output Viewer".
Other "must have" for your purpose could be the "Syntax Highlighting" and maybe othes and "Code Completion", "SyncTeX", etc.
Among these, Gummi could be a good option because is very simple, but unlike many others editors with built-in viewers, can be configured to compile automatically every x seconds, so that the changes in code are showed almost in real time with small documents. But feel free to compare to others of the list. In most cases is free.