He doesn't say that the fact that Israel investigates itself should let it off the hook. He says that in cases where the report says that Israel deliberately targeted civilians, it didn't actually do so, but that the death of civilians was due to mistakes. Why don't you read first what he wrote in "The Washington Post" before you comment?!
Guatemala judge orders detention of president in corruption scandal (Reuters)
from the article: Hamas: Goldstone retreat doesn't negate war crimes committed in Gaza