What do Christian Pacifists believe is necessary for a killing to be justified? Christian Pacifists, for example Mennonites, are often portrayed (and indeed frequently portray themselves) as believing that violence, especially lethal violence, is never justified.  I am sure this is not actually the case, as God explicitly causes the death of several people in various circumstances in the Bible.  Furthermore, God orders the Jews to kill people at various times for various reasons, and seems to condone and approve of killings such as, for example, those done by Esther, where the killer is not visibly ordered to perform the killing by God.  While I'm sure there is an explanation, I am unsure what it is, so:

When (i.e. in what situations) do Christian Pacifists believe killing is acceptable and when do they think it is actually good and why do they think these things, keeping in mind the various justified killings we see throughout the Bible and Sacred Tradition.
All the examples you are asking about are from the Old Testament. Fundamentally it is important to realize that Christianity is not the same a Judaism. The coming of Jesus Christ ushered in a New Covenant, and a new way of doing things. Many things in Christianity are changed from Judaism, and according to the Peace Churches, pacifism is one. Just as animal sacrifice was a necessary 'temporary measure' until the real sacrifice was made, so are the killings of the Old Testament a necessary measure until the New Law could be brought into place. The events you describe in your first paragraph take place under the 'old rules'.

To answer the question in the second paragraph, most Christian pacifists believe that it is never right for a Christian to kill. Most also believe that no forms of violence are ever right for Christians. Many would believe that it is acceptable for the state or government to kill as part of its job of lawfully protecting its citizens, but that it is not right for Christians to take part in that process. Many of those churches also believe in separating themselves very clearly from the state, so that they can never be associated with any state-sponsored killing.