The Rules Of War

 I really recommend reading Robert Farley's explanation of what the Law of Armed Conflict is about. The basic idea is that there is "just war" (jus ad bellum) and "justice within war" (jus in bello). The interesting point he makes is that there are just wars - clear wars of self-defense would qualify - but there is also lawful conduct within wars and it's really important that they remain separate.

Why? Hamas believes its cause is just, given the treatment of Palestinians by Israel. Israel believes its cause is just, because every country has a right to security and self-defense. 

However, Hamas believing its cause us just (jus ad bellum) does not give it license to kill civilians in violation of jus in bello. The similar restraint applies to Israel. They have a right to self-defense, but they have nuclear weapons and could simply erase Gaza from the face of the earth. 

Those that argue that the justice of the Palestinian cause allows Hamas to violate jus in bello are basically denying that jus in bello exists and if that is true, why should the IDF abide by the rules of war regarding civilians. If the justice of your cause means you can just kill civilians at will, then that applies to both sides. 

This is, of course, a nuanced and subtle distinction that I'm sure will convince people on social media.