What do we mean when we say it today?
This is one of the most iconic quotes of King Lear. It's a big deal. So it's no surprise that it's re-quoted a bunch of times. Usually, people know it's Shakespeare when they bust this out. It's just got that ring to it. The truth is this phrase means the same thing now as it did when Lear shouted it from the rooftops, er the heath.
It's about feeling like everyone else is out to get you. Even if you've done some stuff that's not so good, it's okay if others are doing stuff that's worse. To recap: "it's not me, it's you."