Neither of your arguments has any valid basis.

1a A person who is not an active soldier/ combatant is not a valid military target under any international treaty on war. The very idea that a person who once served in the military but was no longer serving would be a military target would have destroyed the concept of civilian as when those treaties were written mandatory universal male conscription was the norm.

1b In any case Hamas and Gazan "civilians" indiscriminately slaughtered, kidnapped and raped anyone they came across. This included children, Thais, Israeli Arabs, many foreigners and many Israeli Jews who had never served in the IDF.

2 Israel does not say that all adult Gazans are valid military targets.

Hamas turned almost every building in Gaza into a military target by placing weapons inside and military tunnels underneath and uses Gazans as shields.

Israel attacks legitimate military targets and civilians are killed because of Hamas's actions. Civilians are not the targets and Israel is far too careful to protect Gazan civilians at the risk to its soldiers and the mission.