This was spoken by Jesus and unless I am mistaken, the Muslims do not accept Jesus. Yes we are saved by grace and not by works, but that is through the blood of Jesus. Sorry, but unless this Atta had a last second conversion, we can pretty safely assume he isn't enjoying Heaven right now.
In the Catholic view there is such a thing as "invincible ignorance." If Atta really and truly, with every fibre of his being, believed that to be a Muslim and die for his faith, to hate Christians and do the killing that he did, was pleasing to God in every way. I don't think he would go to hell. If he truly thought that to be a Christian would separate him from God and he feared that separation from God, then I don't think he would go to hell.
Of course everything he believes is OBJECTIVELY wrong and always will be. But only God knows his heart and the kinds of things that influenced him to take the path he took.
Then again every person is born with "law of God written on their hearts." Everyone has an obligation to find the truth and form their conscience in the light of Gods laws. If Atta, filled with hate, knowingly embraced a God of hate for the sake of hate, then he would violate the law that God had written on his heart. And his guilt would be very great. In other words if Atta rejected the good, on purpose, after serious reflection, he's in lots of trouble.
But Atta aside, everyone who is saved is saved by Jesus death on the cross. But that does not mean you have to know who Jesus is. It mostly means he has to know who you are. Jesus said "FOLLOW ME." He can see in your heart and see by your life if you "followed him" or not even if you never heard of him. Anyway that's how Catholics see it.