It really isn't all that hard to separate demonization from valid criticism. Broad, sweeping statements; misusing terminology; parroting soundbites; victim-blaming; emotive language; false equivalencies. All demonization. Examples used against Israel in this thread alone: genocide, murder, mass murder, terrorist, ethnic cleansing, herding, indiscriminate bombing. Demonization.
Valid criticism comes from knowledge of specific events, use of proper definitions, and a measured argument from facts. As an easy example: indiscriminate bombing. Hamas fires rockets indiscriminately, because their devices have no ability to target, and often fail or fall short. To say that Israel bombs "indiscriminately" is not factual. Israel is capable of gathering intelligence, communicating with people on the ground, and very precise targeting (though mistakes still happen, and equipment still fails to perform, people aren't where they said they would be). Valid criticism of Israel, therefore, would not include a claim of "indiscriminate bombing". However, let's say Israel has intelligence that the cell phones of ten very prominent Hamas members were all gathered close together near the Rafah crossing, in close proximity to civilians. Were Israel to precisely target those ten Hamas militants, knowing that there would be incidental harm to the surrounding civilians, you could fairly criticize that decision. Valid criticism.