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.