To me, naturally, it was murder. Yet I can accept the term "execution" too, even use it sometimes. The biggest difference from the murders above to me is especially the fact that those men were all pretty much "ambushed". What happened with the Romanovs had many features of "execution". They were prisoners. They were officially enemies of the people. According to what we know they were informed about their imminent death, albeit mere seconds before it. Their deathwas decided by one or more people in power - that means the decision was within their legal rights (as sick as that sounds). None of this plays any part with Lincoln, Kennedy or King.
The Romanovs were murdered. The Romanovs were executed. Both are correct.
1. The fact that they were first kidnapped instead of being "ambushed" does not add any legitimacy to the murder.
2. What does it mean that they were "officially enemies of the people"?
If there were "enemies of the people" in Russia in 1918, they were the band of revolutionaries that reached power after a coup in November 1917 and imposed terror, war, famine, misery and religious persecution on the Russian people. And where does "officially" come from? According to what law? According to what verdict?
3. "Their death was decided by one or more people in power - that means the decision was within their legal rights (as sick as that sounds)."
The Holodomor (the man-made famine which killed millions in Ukraine in 1932-33) was decided by one or more people in power in the Soviet Union.
The Holocaust was decided by one or more people in power in Germany.
The massacres in the "killing fields" were decided by one or more people in power in Cambodia.
The bombing of the Pan Am Flight 103 was decided by one or more people in power in Libya.
Gruesome beheadings in Irak and Syria are decided by one or more people in power in what they call "Islamic State"...
If a group of terrorists gain control of a building or a plane and start murdering hostages, they are not acting "within their legal rights". Sometimes terrorists gain control of a whole country. They did in 1917.