Would breaking a baseball bat be the result of a good swing, a bad swing, or either?

Bad swing.

neither. the bat will break if hit well enough on the same way that the wood is cut or umm.. just look at the bat and try not to hit the ball where the wood pieces go horizontally :)

It breaks most often when you hit a pitch that's on the inside. Because that hits the thinner part of the bat so it's easier to break

Neither.

All bats possess what is called vibrational modes and the ideal mode is called the sweet spot.

Here, in this mode, no vibrations are transferred to the batters hands when the ball is well struck. All of the bats energy is transferred to the ball and consequently it is where all ball players love to hit the ball.

Down towards the knob, however, the bat is much thinner and if the ball collides with the bat in that area, it sets up tremendous vibrations in the bat which are transferred to the hands and if hit squarely will sometimes distort the bat to the point where the wood fibers lose their elasticity and the bat will shatter if it is maple. or just break off if an ash bat is mishit.

To understand all this, I suggest you pick up a copy of "The Physics of Baseball" and beginning on page 121 and continuing through page 130, there is a comprehensive and detailed analysis of this phenomenon.

The book is only $ 5.00 and is available online or at brick and mortar stores.

A broken bat is usually the result of poor wood. The bats are constructed to have a large head and a very slender lower handle. This will increase the bat speed, and result in a lot of broken bats.
I played baseball for 18 years and broke ONE baseball bat.
I played pro ball in class A and we were assigned 6 baseball bats for the season.

The bats shatter now days usually on inside fastballs and a late swing.

Basically, it is poor wood.

A defective bat! or a bad swing!