"Radical simply means ‘grasping things at the root.'" - Angela Davis
It's common practice among those of us outside Israel who have been frustrated by the hostility and intimidation we encounter whenever we voice criticism of Israel to point to the fact that there is greater freedom to criticize Israel in Israel. "Look at the critical articles published in Ha'aretz," we will say. "Look at the Israeli peace movement. Look at Peace Now and Gush Shalom." Tactically, this is a useful point to make in an argument. I know because I've used it myself.
The truth is, unfortunately, that this much vaunted criticism within Israel - by the liberal media, by the so-called Israeli Left - is overwhelmingly inclined to blame the oppression of Palestinians merely on specific leaders or policies. Uri Avnery, for example, the founder of Gush Shalom and one of the most far-left public figures in Israel, writes a regular syndicated column in which he blasts the brutality of this or that general, the cruelty of this or that politician, the unfairness of this or that law. He's often quite incisive and witty. Avnery, like most of the Israeli left, is a Zionist - a critical one, to be sure, but a Zionist nonetheless who believes that a good movement has been corrupted by bad leaders and who periodically scans the horizon for the leader who can finally set Israel on its righteous path. 
Israeli violence and oppression, however, is rooted not simply in a few laws or politicians, but in the ideological foundations of the state itself. The problem, in short, is Zionism. Any opposition to Israel rooted in Zionism can only seek to mitigate Israeli apartheid and racism, not end it, because apartheid and racism are what Zionism - and by extension, the Israeli state - are all about. Zionism is rooted in the fundamental premise that the state be a Jewish state and that it occupy the physical space of an ancient Arab Christian and Muslim culture. Because it is impossible to achieve these two goals simultaneously without violence and racist oppression, you cannot have a genuine peace movement that is Zionist.