Pedro I was the eigth king of Portugal, he was born in Coimbra on April 8, 1320, and died in Estremoz, January 18, 1367; he was nicknamed The Cruel', 'The Just' and 'The Vindictive' because he ordered some of his enemies to be killed in a gruesome manner and just one survived.
Many of the events related to the murder of Pedro's second wife are
legendary or have been reinterpreted for several reasons thorough many
centuries; even artists have been inspired by the story, so the details
might not be accurate. The essence, however, is that Pedro, then a
prince going to be the heir of a kingdom not only lost a person whom he
loved, but also his leadership was questioned right before the beginning
of his rule. He had to retaliate.
While politics have always been dirty, in past centuries being born a king or a noble person wasn't a guarantee of anything. You had to be more ruthless than anyone else, just in order to stay alive.