Is it wrong to judge how people behaved in the past based on today's morals?

The piece of paper is not the law.  The piece of paper is the means by which 'the law' is codified and disseminated for all to see/read.

"

Law is a system of rules that are created and enforced through social or governmental institutions to regulate behaviour. Law is a system that regulates and ensures that individuals or a community adhere to the will of the state. State-enforced laws can be made by a collective legislature or by a single legislator, resulting in statutes, by the executive through decrees and regulations, or established by judges through precedent, normally in common law jurisdictions. Private individuals can create legally binding contracts, including arbitration agreements that may elect to accept alternative arbitration to the normal court process. The formation of laws themselves may be influenced by a constitution, written or tacit, and the rights encoded therein. The law shapes politics, economics, history and society in various ways and serves as a mediator of relations between people."

What does this have to do with atheism? or otherwise?