In one sense, I don't think we should be "pleased" that he is dead. God is not "pleased", which should be our ultimate example:
Ezekiel 33:11 (NIV)
11 Say to them, ‘As surely as I live, declares the Sovereign Lord, I take no pleasure in the death of the wicked, but rather that they turn from their ways and live. Turn! Turn from your evil ways! Why will you die, people of Israel?’
However, just because we should not take pleasure in his death, does not mean we cannot be pleased with the net outcome of the situation. We can be pleased that future lives will be spared.
For example, even though God did not take pleasure in the death of the wicked, he still ordered sentences of death for certain offenses (and he does all that he pleases). This was for the net benefit of the society. So, although God did not take take pleasure in the death of the individual, he took pleasure in the protection of the masses.
So while we ought to grieve his individual death, for his sake, we can still be pleased overall for the sake of others.