The people who killed them, whether they are Israeli or Palestinian. How is this something that people disagree about?
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.
That doesn't justify attacks on innocent civilians in Israel, and those attacks don't justify attacks on innocent civilians in Gaza.