Terrible code is not a good excuse to be mean, because terrible code is not well-correlated with having been written by terrible people.
When you feel the need to write a comment, first try to refactor the code so that any comment becomes superflous.
Eagleson's Law: Any code of your own that you haven't looked at for six or more months might as well have been written by someone else.
When you come across a stumbling block because the code doesn't quite fit anymore, or you notice two things that should really be merged, or anything else at all strikes you as being "wrong", don't hesitate to change it. There's no time like the present.
It's not at all important to get it right the first time. It's vitally important to get it right the last time.
Refuctoring - the process of taking a well-designed piece of code and, through a series of small, reversible changes, making it completely unmaintainable by anyone except yourself.
DRY ‐ Don't Repeat Yourself ‐ Every piece of knowledge must have a single, unambiguous, athoritative representation within a system.