From wiki.

The international community considers the establishment of Israeli settlements in the Israeli-occupied territories illegal under international law, because the Fourth Geneva Convention of 1949 prohibits countries from moving population into territories occupied in a war. Israel maintains that they are consistent with international law because it does not agree that the Fourth Geneva Convention applies to the territories occupied in the 1967 Six-Day War. The United Nations Security Council, the United Nations General Assembly, the International Committee of the Red Cross, the International Court of Justice and the High Contracting Parties to the Convention have all affirmed that the Fourth Geneva Convention does apply.

Numerous UN resolutions have stated that the building and existence of Israeli settlements in the West Bank, East Jerusalem and the Golan Heights are a violation of international law, including UN Security Council resolutions in 1979, 1980, and 2016. UN Security Council Resolution 446 refers to the Fourth Geneva Convention as the applicable international legal instrument, and calls upon Israel to desist from transferring its own population into the territories or changing their demographic makeup. The reconvened Conference of the High Contracting Parties to the Geneva Conventions has declared the settlements illegal as has the primary judicial organ of the UN, the International Court of Justice and the International Committee of the Red Cross.

And all that is aside from the human cost to the Palestinians of the usurping of their land.