Why was marrying brother's wife unlawful?

In Mark Chapter 6, verses 17 - 18 John the Baptist had told King Herod he couldn't marry his brother's wife.
How was that ever unlawful? Assuming his brother was dead.
Why was John interfering with the king's matters anyway? Seems a shame to die for such a matter. Just clarify please.

Herodias was married to Herod II with whom she had a daughter, Salomeea (the girl who danced at Herod Antipas's birthday party). Herodias divorced her husband (he was still alive) and married Herod Antipa, the stepbrother, because he was wealthier and inherited dominion over Judeea (unlike Herod II, who was removed from the succession).

Thankyou.
So was this illegal or immoral or both?

Yes, both. And everybody knew, but only Saint John the Baptist had the courage to criticise them openly.

He wasn't dead. It's that simple.

So was divorce not acceptable?
Forgive my curiosity.

Leviticus 18
16You must not have sexual relations with your brother’s wife; that would shame your brother.
18You must not take your wife’s sister as a rival wife and have sexual relations with her while your wife is still alive.
and with regard to Salome
17You must not have sexual relations with both a woman and her daughter. You are not to marry her son’s daughter or her daughter’s daughter and have sexual relations with her. They are close relatives; it is depraved.

I not sure if any of these reasons were acceptable grounds for divorce, that is a good reason to divorce someone in order to do them.