"Israel not only has the right to defend its citizens, but the duty to do so according to international law. The military operation is legal. In this context, the committee found that Hamas and other organizations had committed war crimes against civilians in southern Israel. The rockets terrorized women and children every day. This was mentioned in the report in detail," the judge argued.
Emphasis added. It's a little unclear, but Judge Goldstone has persistently sought to distinguish the jus ad bellum aspects of the conflict (the decision to go to war) from the jus in bellum aspects (how the war is prosecuted), claiming his mandate was only to investigate the latter. Of course, he is perhaps the only person who has held to that distinction in the ensuing debate over the controversy. I think the contours of how Goldstone's report was received would have changed dramatically if he had written: a) Israel unambiguously was within its rights to launch a military strike into Gaza in response to rocket attacks, and b) the manner in which the campaign was prosecuted at times amounted to war crimes. Part "b" of that formulation came through quite prominently, part "a" not at all.
But the passage above is still slightly opaque, so I don't want to say with total confidence that Judge Goldstone is endorsing the jus ad bellum legality of Cast Lead.