Is it bad to know thyself in the extreme?

No.

You’ve got it all wrong.

There are three Delphic maxims, and you’ve read the second one as if it was a qualifier on the first, which it is not. It does, however, tend to depend on the first. That is, you might want to begin by knowing yourself.

1) Know Thyself
2) Nothing In Excess
3) A Pledge Brings Trouble

Well, I believe you ought to know yourself well. I’m not sure if you could know yourself in the extreme. But if you did know yourself in the extreme and didn’t like that person, then what?

Are you talking about me again? You are such a jerk!

This discussion is about you, not to you. Butt out.

What does it mean to know thyself in the extreme? Is ignorance bliss?

Probably best not to know. In certain circumstances you might behave quite differently from the usual you.