This is a deeply flawed analogy. We are not talking about the actions of Israel but the Response of Iran being illegal: targeting civilian targets, forbidden by the rules of war.
A better analogy would be: The law says a criminal should be punished with a maximum of 5 years in prison, but the judge hands out 20 years.
What the judge did was still wrong and illegal, no matter what the criminal did.
If you want to live in a world where everyone can bomb everyone, just remember why we introduced those rules and how many nations have nukes today.