Which part of "Thou shalt not murder" don't you understand?

What part of "Thou Shalt Not Kill" do you find difficult to understand? It may have been legal but legality does not mean morality. Humans have legalized immoral behaviors for as long as humans have existed.

He didn't physically chop anyone's head off. Anne Boleyn and Catherine Howard were both charged with treason, tried on that charge, found guilty and executed. Adultery by the Queen was treason and the punishment, for a woman, was to be burned at the stake or beheaded as the King decided.. Those accused of adultery with the Queen were also tried and condemned.

George Boleyn, Sir Henry Norris, Sir Francis Weston, William Brereton and Mark Smeaton were all found guilty of treason for having had sexual relations with Anne Boleyn and were all sentenced to the traitor's death of hanging, drawing and quartering. The King reduced the sentence to beheading and they  were all executed on Tower Hill on 17 May 1536. 

Thomas Culpeper and Francis Dereham were charged with treason for having sexual relations with Catherine Howard. Both were sentenced to be hanged, drawn and quartered but the King remitted Culpeper's sentence to beheading. Dereham suffered the full penalty.

It is likely that Catherine Howard was guilty of adultery and also likely that Anne Boleyn was not, but it is impossible to prove either way today.

Henry was acting according to his conscience and with the support of the law and the church so he was not offending his god and therefore not sinning.

Not according to the new Religion he created so he could do that and get remarried.

King Henry the Eighth was a mass murderer both of his wives and in his religious beliefs. 

I'm King and I don't want any Pope telling me what to do.

I'm King and I am going to steal all the money lands and authority from the RC church. 

He made himself the head of his church, so he could do what he wanted.

Yes  and it was appalling

Many a sin has been committed by one who's power can't be safely challenged.

Consider Herod.

That's what happens when God grows tired of that sort of thing.

It is said that Henry was literally very sick.

He had two of his wives executed - he did not attend, we are told. 

Is abuse of power to the point of murder a sin? What do you think?!