I mean, Israel now is located on land that was partially owned by Palestinians. Many Palestinians were driven from their homes, partially by various Arab leaders of the region, but also by Israel itself. The Palestinians certainly have a legitimate grievance with what happened in 1948.

In addition, a lot of the occupied territories are effectively lands in which Palestinians are to some extent "imprisoned", neither having their own country, nor being granted any kind of citizenship. These lands were won in a war, true, but the standard course of action is either to return them, or to annex them, neither of which Israel has chosen to do (for obvious demographic reasons). But the end result is a situation in which millions of Palestinians were driven out of their home in 1948, and are now refugees living in territories with no self-determinance.

Israel isn't without a complicated history and a complicated existing situation. The same can be said of many countries, btw.