And they're 100% right.
International law is very simple and straightforward - land forcibly annexed is not legitimate. Israel has no legal or valid claim to any territory beyond what it was granted in UN Resolution 181 in 1948. Every single inch of land it's taken since then has been taken illegally, and in direct violation of the rights of the inhabitants.
And that's even before getting into the overt evil that Israel has committed along the way - the terrorism and brutality and oppression and murder that it's employed to incrementally steal the rightful territory of the Palestinians.
And yes - the Palestinians have every right to stand against Israel's brutal theft of its territory and murder of its inhabitants. Their resistance is not only morally right, but entirely in keeping with international law.