Good King Wenceslaus was a Duke of Bohemia, Wenceslaus I, whose name in Czech is Vaclav, which is pronounced “VA-tslav,” because the letter C, without a diacritic, is pronounced “ts” in Czech. Wenceslaus is the Polish spelling, and it’s worth noting that “Czech” is also the Polish spelling of Czech. In Czech, it is “Čech.”
This Duke of Bohemia was posthumously promoted to king by the Holy Roman Emperor Otto I. King Vaclav was called “Vaclav the Good,” because he was both a generous, merciful ruler, and pious. However, I think “King” was just honorary, and Bohemia remained a duchy until the 1100s.
He was assassinated (by his brother) and considered therefore a martyr and was canonized. He is the patron saint of the Czech Republic, and Vaclav has since been a popular boys’ name, remaining so throughout the “Czechoslovak” Communist period, so that the last president of Czechoslovakia, and the first of the modern Czech Republic was named Vaclav Havel.
My mother spoke Czech and Slovak both fluently enough to be published in both languages. She met both Vaclav Havel and Bill Clinton at some event she was invited to, because it had something to do with Czechoslovakia, and she was a nationally-regarded expert on Czech, with the bonus of speaking Slovak as well.
This stuff was dinner table conversation.
Safe to say, the Good King never massacred anyone.