What do you think world leaders should've done differently?
Send all collaborators to the international criminal court. (In the US, especially congress critters and members of the executive branch.)
Also, blockade Israel proportionally to the Gaza blockades. I doubt they would have kept murdering red cross members in Gaza once the supermarkets started running out of food.
If you had to guess, what do you think they might do? I feel like questions like this are in bad faith. Think about it for a minute, and try to come up with some things that they might do.
The post you are responding to lists several possible actions. Another good start would be to stop providing the weapons that have done this damage.
Cut off weapon gifts+sales to Israel, impose sanctions. Same way we dealt with the South African apartheid state.