Alright this might also give you an insight into why apostate was executed.

In Islam obey Allah included obeying the Prophet. The Quran and Islam served as the constitution of the community. No separation of state and religion. When you insult and denounced the prophet you are considered to be committing treason. Not only are you turning your back on the Prophet but also God , the people and also your community members. A traitor can put your whole community at risk.

It’s no different then committing treason in any country. Betraying the constitutions and undermining the rules and regulations of any country would land you in serious consequences. Case and point Edward Snowden.