For your information it was Israel's Hannibal protocol where they are ordered to eliminate any of their forces who are left behind. What you call "civilians" (in a concert in the middle of a war where everyone else is heading to bunkers) were recruits of the Unit 8200, a subordinate to Aman the military intelligence directorate. In simple terms "spies". This particular unit is responsible for orchestrating terrorist attacks overseas.
IDF was ordered to eliminate them before they could be caught by Palestinian forces and their classified information be extracted. Something they failed to complete and the acquired intelligence has already helped neutralized countless terrorist cells across West Asia.
As for Hamas, for your information according to international law and even the recent International Court of Justice it is a right for any people who are subject to invasion, apartheid and genocide to arm themselves and start defending with any means possible.
You may also find it interesting to know that the globally regarded icon of democracy and social justice, receiver of more than 250 honors, including the Nobel Peace Prize, Nelson Mandela and his ANC (African National Congress) is also categorized as a terrorist organization by the United States regime. Why? Because like H.A.M.A.S. (Arabic acronym for Islamic Resistance Movement) they were also fighting against apartheid and genocide that US regime supported!