Thebes isn't a democracy. Creon is king and what he says goes. He's given a very specific order, and he expects it to be obeyed without question: Polyneices's body must be left to rot out in the open, and woe betide anyone who tries to bury it. Creon, like Antigone, also believes that he's faithfully carrying out the will of the gods. Polyneices, along with the other members of the so-called seven against Thebes, had desecrated the city's sacred temples during the recent civil war. As a result, Creon genuinely believes that the gods are on his side and that they approve of Polyneices's punishment. He's also very much a man of his time in that he expects absolute obedience from his son, Haemon. King or not, Creon is regarded by the prevailing standards as lord and master in his own house. This entitles Creon to insist on Haemon's compliance with his orders to the letter.

Creon sums up his ruling principle when he states:

The man who considers more important
than his fatherland his friend, I think him
worthless.

In other words, Creon puts nationalism or love of homeland ahead of family and friends. He is a tyrant rigid in his avowed principles, and lacks the capacity to see life from other people's point of view. This brings him into direct conflict with Antigone, whose principles clash with his: she puts the humane values of family and common decency (burial of a family member) ahead of rigid obedience to a ruler. Creon simply cannot tolerate this, saying:

She showed
herself capable of insolence then,
going beyond the laws put before her. 
Her second insolence, after she had
done it, was to exult in her deed and
laugh that she had done it.

Creon proves himself an unwise ruler, a typical tyrant with no sense of humor, unable to be flexible. He is too fixated on himself and his own ideas to rule well. In reality, he puts his own needs to be obeyed and affirmed ahead of the needs of the state, ironically undercutting his avowed fidelity to the state. This is a factor leading to ruin and tragedy for him.

Interesting question. By "principles," I assume you are referring to his values or morales which dictate how he acts and what he says and does. One of Creon's first speeches directly refers to his "principles," which also explains why he has refused to allow the body of Polynices to receive proper burial. Note what he says:

These are my principles. Never at my hands

will the traitor be honoured above the patriot.

But whoever proves his loyalty to the state--

I'll prize that man in death as well as life.

Of course, herein lies the conflict of the play, as he ignores the "unwritten laws" that Antigone insists must be upheld by giving her brother proper burial. We can therefore infer that Creon's "principles" involve clinging on to power, whatever the cost, and harshly punishing any form of rebellion, even if that brings him into contact with his own family or the Gods. Creon, by clinging firmly to his principles throughout the play, therefore sows the seeds of his own destruction and in the end loses everything. He remains an important lesson for any would be despot about the dangers of authoritarian power.