"Father, forgive them, for they know not what they do."
First, I think forgiveness is very hard.
Second, how the heck can you know what these stupid men thought seconds before their deaths. Remember, supposedly only one or two on each plane knew what exactly was happening. The others thought they were hijacking the planes and found out moments before their own deaths. Perhaps they did ask for forgiveness seconds before their deaths. They can't really "show us any signs" of being remorseful, can they?
Also, the Pope is from a religion that believes in Purgatory. Whether you believe it or not, he does believe in it. That's why the Pope can ask God to forgive.
God is kind and merciful, He is also just.