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.