The answer is easier than we imagine..
Allah say in Qur'an: ".. if any one slew a person - unless it be for murder or for spreading mischief in the land - it would be as if he slew the whole people: and if any one saved a life, it would be as if he saved the life of the whole people." Surah 5: Verse 32
And the punishment of those who kill innocents and terrorize safe people in Qura'n is: "The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter" Surah 5: Verse 33
And Prophet Muhammad -Peace Be Upon Him- said: "Whoever kills a person whom we have a treaty will not even come closer enough to Paradise to smell its scent." Sahih Bukhari
Usama Bin Laden committed and incited to kill innocents around the world. A just punishment for him is the "qisas", who kills must be killed. The world has get rid of his evil.
That's why we shall cheer up. When a criminal is punished, we cheer up. When justice is applied, we cheer up.
One mega criminal has been killed, but there are still tens of butchers who must be captured and be presented to a just trial. Ghaddafi, Sharoon, Olmert, Livni, Basshar, Mubarak, Habib al-Adly, etc.