Because shit like this happens every goddam time.
A pacifist cannot easily claim that without war, everything will be just fine. Jesus said "He who lives by the sword dies by the sword," shortly before being violently killed (many of his nonviolent followers were also violently killed; it was some time later that his followers quit practicing nonviolence). The world, in addition to beauty and love, is filled with evil and brutality. A pacifist does not look at the violence in Libya with blindness toward its awfulness, and does not suffer the illusion that Libya's problems will go away if left alone. But a pacifist knows not only the moral problem of attempting to use violence to stop violence, but also knows the practical problem of humanity, of atrocity, of blood.