Broken kapala as a sign of what? It was said in a book by my spiritual master that 'breaking unloyalty to one's spiritual master is worse than breaking a kapala', of which I deduce that breaking a kapala is a really bad sign.

Is there any root text explaining what is particularly bad about breaking a kapala and what is the meaning of it?
Just a "non-scholar" and principle answer.

Kapala, used here (Cambodia, spelled 'kpal' in Khmer) as word for head.

If one cuts of oneself the a/the master - in regard of the Buddha here, or what ever transports his way - is therefore more worse than to throw away a life by breaking the kapala because another live is secure, a good or better seldom and to meet the Buddha and/or his guidiance(s) very, very seldom.

When we remember even the Buddha, not even he break up or had been unloyal to his previous teachers.

That is also why the wise say:


  "Now what is the level of a person of no integrity? A person of no integrity is ungrateful &amp; unthankful. This ingratitude, this lack of thankfulness, is advocated by rude people. It is entirely on the level of people of no integrity. A person of integrity is grateful &amp; thankful. This gratitude, this thankfulness, is advocated by civil people. It is entirely on the level of people of integrity."
  
  Kataññu Suttas: Gratitude


While the parents are called the "first Gods", followed by those who give skills (normal teacher), those giving guardians toward heaven or beyond are the highest gods one should wisely honor and venerate.

Where would one headless run around if not have been supported by them?

The lesser such values and the lesser such as right view and gratitude the more near it runs in direction of communism and there most worse breaking of heads.

So ungreatfullnes and lack of gratitude for what serves such leads itself to ones breaking of the kapala, anyhow personally and sometimes at large as genocide, killing then the current gods, parents, masters away.

But it is not just physical to understand. Most having done a very bad deed in regard of gratitude suffer sooner or later as if the head is broken or spitted.

To Kesi the Horsetrainer

One does good to "kill" the Demon Kesi, the childhood disease, in one self, when ever he arises.

It's good to take as much as possible care never show signs of ingratitude in regard of what possible needs regards generally.

At least hete a very useful talk on gratitude and going beyong: Karma &amp; Gratitude.

[Note: This is a gift of Dhamma and not meant for commercial purpoes or other gains in the world]