Steve: "I'm not denying that there is, or has been, a considerable Hamas presence in the hospital. That would be a war crime. So is the deliberate targeting of a hospital by the IDF."

Once you have accepted that Hamas have (or have had) a considerable presence in the hospital then the war crimes are those of Hamas.

From The Guardian

Article 8 of the Rome statute, which established the international criminal court (ICC) in The Hague, defines a long list of war crimes including “intentionally directing attacks against buildings dedicated to religion, education, art, science or charitable purposes, historic monuments, hospitals and places where the sick and wounded are collected”.

But it makes an exception if the targets are “military objectives”. Philip-Gay said that “if a civilian hospital is used for acts harmful to the enemy, that is the legal term used”, the hospital can lose its protected status under international law and be considered a legitimate target. Nevertheless, if there is doubt as to whether a hospital is a military objective or being used for acts harmful to the enemy, the presumption, under international humanitarian law, is that it is not.