You wrote: “ Is there any justification to stand by idly when someone's life is in danger? No! But what if you have to cause harm to prevent harm?”
You agree with me that we are not allowed to kill other humans for survival. The moral principle you quote would in fact require us to step in and save someone’s life, who someone tried to kill this individual for his/her own survival. In order to not choose a speciecist position, one must also extend this to all other animals, and not stand idle when their life is in danger and to not oneself take their lives for ‘taste’, habit, convenience, tradition or survival. This includes the 58 billion land animals killed in the world each year for food, and the more than 1000 billion marine animals killed for food each year. In order to not participate in the crime and not to stand idle when this happens one is obligated to go vegan and to help others to understand the importance of going vegan.
This shows that to look at the primates for moral guidance to determine what to eat doesn’t hold, and people consistently are applying their eating habits based on what other primates eat, would endorse cannibalism as well as killing and eating other animals both for ‘pleasure’ and survival. There are exceptions though among the primates. The gorilla, an herbivore, eats in an ethical way.
“Nonhuman animals have an interest in not being treated as human resources just as humans have an interest in not being treated as the resources of other humans.” [Quote: http://www.abolitionistapproach.com/resolution-2-for-2010-stop-vegan-bashing/#.UZ5upZxc18E ]
“Speciecism is wrong because it’s like racism, sexism, heterosexism, etc., which also involve focusing on an irrelevant criterion (race or sex or sexual orientation or whatever) to justify not according equal consideration.“
The other points you bring up is subject for another moral discussion, which is whether violence can ever be justified, e.g. using violence and killing someone as an act of ‘self defense’. I have argued that it is most of my life, but I changed my position some time after becoming a vegan.
The first part of this article has good arguments for nonviolence: http://www.abolitionistapproach.com/a-comment-on-violence/#.UZ5P05xc18E