Yes, we can say the events of September 11th 2001 were truly evil. I don’t believe for one second that it could be otherwise. But, if morality is subjective, then the events that took place sixteen years ago on September 11th are just a matter of taste.
You can’t say these events were objectively evil, wrong, or unjust. They are just someone else’s moral opinion as opposed to yours.
The morality of the terrorists, no matter how much you may dislike it, just evolved differently than yours.
Because Morality is not Reality on atheism.
The fact of the matter is that the events of September 11, 2001 were evil, and everyone knows it. Evil assumes a standard by which we measure behavior. Our reaction to this tragedy confirms that it is not right.
You can only know objective evil in a theistic worldview; you can only know objective right and wrong if God exists. This is in stark contrast to atheism where all you are left with is your opinion that has evolved over time; not right, not wrong, just an opinion.
Most atheists agree that September 11th was wrong; what they can’t do is ground their claim.
Where is this standard for good and evil on atheism?
Add to that, the inability of atheism to account for the immaterial conscience, and now morality is nothing more than a molecular reaction. You can only know evil if there is an objective standard outside of yourself that confirms what you know internally; only theism can account for both of those realities.
As an atheist, can you really say the events of September 11th, 2001 were evil in any meaningful sense?
Truly Evil
On theism, the events of September 11th, 2001 were truly evil, wrong, and unjust. The perpetrators of these crimes will be held accountable for their actions and true justice will be served.
Our hearts and prayers go out to all of the families and loved ones effected by this horrible tragedy.
But this is only true in a theistic world.
Not so for atheism because: Morality is not reality on atheism.
Author:
Anthony Uvenio is one of the directors of 
New York Apologetics along with Nick Mitchell. He serves in his local church and seeks to equip people to engage the culture with the truth claims of Christianity. You can watch him debate an atheist here: DEBATE