Question: if a man was coming toward my son to try and harm him, would I fight him to protect my son?
Answer: if I was incapable of running away, yes, I would fight to protect my son.
Question: if a man was coming toward my son to try and harm him, and he was running through a crowd of innocent people, would I throw explosives into the crowd to prevent the man from hurting my son?
Answer: No: while I would justly defend and protect my innocent son, I should not commit a violent act that harms other innocent people in the process.
And this is a part of my pacifism. It is not that I believe limited violence for self-defense is wrong. It is that war is a larger-scale level of violence that always leads to innocent people being killed and harmed. If you are willing to support a war, you are de facto supporting innocent people being hurt: that is simply the nature of modern warfare.
Great human suffering is an inherent aspect of war. War must always be an absolutely necessary last resort; it should never be entered in choice.