Then let me address these question by question.
If mankind is indeed aggresive, savage, brutal, and also violent...why would you adopt such philosophical beliefs of non-violence or non-resistance?
Probably for the same reason one would advocate self-control: because we want to realize our rational nature and trascend the basic instincts and urges of being mere animals.
Would it be better to adopt a certain philosophy base on self-defense rather than take a non-violent stand?
I would think so. In certain situations, only harm or violence can stop evil from continuing its destructive path. (Though I will point out that the effects of such defensive measures can be hard to realize before the fact. Case in point- people using extreme measures to protect themselves that drastically effect others for ill, or escalate into worse conflict. That's really another discussiont though) If a gunman is forcing his way into your house, he intends to kill you, rape your wife and children, and steal all your possessions, would you be justified in killing him or maiming him to prevent it? Yes. You could not be considered morally good to stand by and let it happen, or adopt a positon of non-violence if violence is the only option you would have to save what you live for and those you have a duty to protect. Some could argue that you could nonviolently take him down and apprehend him, but that's just ridiculous. Why take that chance? Why risk trying to disarm or judo throw a gunman when you can shoot him or stab him and end it right there?
We could get into "Where does one draw the line", but I think the above scenario is pretty straight-forward: you do what you have to in that case. It isn't like you're being mugged or having your rights stolen; this is far more immediate and personal.
Why would you allow an individual to inflict injury and pain to your body?
I wouldn't. Pain acts as a natural motivator to living beings. We hate it. We naturally try and avoid it. It hurts when you stick your hand on a burner. Why would you cause damage to yourself or allow it when you could avoid it? What is moral about allowing someone or something to harm your body?
What do you really prove by being resistant?Is it some kind of martyr fanaticism that would drive you NOT to defend yourself?
Apparently. It's a belief in honor and a code that trascends mortality and physical being. Faith, really.