It isn’t a defense at all. You said you didn’t understand. Abused people (or peoples) becoming abusive in turn is a common process, and that phrase is commonly used in such a way that searching on it will reveal many studies of the process, and what has been tried (successfully and unsuccessfully) to short-circuit it.

Understanding is not absolution, though. You can both see how the genocide of Jews led to fears, politics, and colonial interventions that contributed to Israel’s genocide of Palestinians, and also condemn that genocide and those perpetrating it (which I absolutely do). But not understanding the motivations makes any problem easier to effectively resist or solve, as you can use that knowledge to determine where and how to apply pressure and create a lasting rather than a temporary change.