This issue hinges on the question of whether we are a society governed by traditionally accepted moral principles or whether we are to accept the secular view that each person should decide what is moral or not based on their own viewpoint (moral relativism).
The first concept allows us to formulate laws and codes of conduct that are simple and easy to understand and obey. The second makes for inconsistency and confusion resulting in a break down of society.
Under moral relativism, Timothy McVeigh was justified in what he did because he felt it was his moral duty, the same with the 9/11 terrorists.
A very slippery slope indeed.