I think we agree that civilian deaths are tragic and should be minimized.

I'm guessing we disagree on who is responsible for those deaths.

The people who killed them, whether they are Israeli or Palestinian. How is this something that people disagree about?

If you attack me and I shoot you, you're to blame for your death, not me.

If a country attacks me and I counterattack to make sure you can't attack me anymore, you remain at blame.

There are details, like deliberate civilian attacks vs collateral, but at the end of the day, the duty of a country is to protect it's own citizens, and not the citizens of the aggressor.

How far back does that go? Does it go back to when Israel the country took Palestinians' property by force?

That doesn't justify attacks on innocent civilians in Israel, and those attacks don't justify attacks on innocent civilians in Gaza.