Since God leaves us to our own devices, who is he to judge us?

God did not just take you to a desrted island and leave you to fend for yourself. "He" created your life; "He" is the one who decides whether you draw your next breath; "He" created the universe and set the standards / rules for living. This is His world, not your's or mine and it is His right to govern it and all people by His ways. If you stick your hand on the lit eye of a stove, well you know the consequences...you'll get burned. Same if you choose to defy the Lord and what "He" says / commands. All of life choices has consequences. Have you not learned that yet?

God is justice, its his very nature. When Adam and Eve sinned, they were separated from God. sin cannot survive in God's presence. Thus God can't let them in heaven. But Jesus died for everyones sins, now if someone goes to hell its because they rejected the free gift.

Almighty God left us all an Instruction manual and free will to decide if we want to follow His ways or not.

He's God.

Yes, it is a parent's job to judge their children - good parents (and God) do.