We get our Sense of Justice through experience and what we are told. If a kid in some remote tribe is told that it is right to murder people and he sees people murdering without remorse his Sense of Justice will say that murder is a moral and just thing to do. So where is the god imposed justice? If god gives out the exact same moral code to everyone then why doesn’t everyone have the exact same Sense of Justice?
Also why would god contradict his own moral code? He says that we shall not murder yet he kills millions of people when he floods the world. And why do we need a Christian god to have a sense of justice? Why couldn't the Zeus or a pink unicorn from Mars impose a Sense of Justice
You are assuming that justice is impossible without a god, but how can you even come close to backing this up? The Bible? The Koran? Both of which are hardly unbiased, not to mention based on events that supposedly happened thousands of years ago. The fact that people have a code of justice that they tend to follow is hardly proof for a god.