Defend the Defenseless

Today I read Proverbs 24 with my family for devotions.
Something that I apparently hadn't noticed before (or forgot about) was verses 11-12:


11 If thou forbear to deliver them that are drawn unto death, and those that are ready to be slain;
12 If thou sayest, Behold, we knew it not; doth not he that pondereth the heart consider it? and he that keepeth thy soul, doth not he know it? and shall not he render to every man according to his works?


I'm not positive yet, but to me, this seems to say you should definitely defend those who are being harmed.
I suppose most people would, but this passage reminds me of a story I read in which a girl was attacked and murdered in a certain city (maybe New York) at night and although people heard her screams for help, nobody came to her aid.  The most common reason given by those who didn't help the girl was, "I didn't want to get involved."  Now like I said, I don't think most people would act like that (at least I hope not), but I wonder what the real reasons someone wouldn't want to help someone who's being attacked.  Here are a few possibilities I thought of: 
Reason #1: The people are just selfish and don't care at all about what happens to others.
They might just not want the trouble of talking to the police or the hassle of any resulting legal issues, but this is also being selfish. 
Reason #2: They're afraid of getting hurt or killed.
This is an understandable fear and reason for being afraid, but if you value the life of an innocent person (presumably innocent, anyway) who needs help more than you value your own, you should not worry about yourself. 
Reason #3: They don't have any real fighting skills at all, don't have a gun, aren't very strong, etc.
Basically, they are afraid they will either get pounded and be unsuccessful in their efforts to save the one being attacked.  In this case, these concerns might also be legitimate.  However, if there is only one attacker, someone who wants to help can simply do all in their power, even if it only consists of yelling and swinging a crowbar or baseball bat.  At the least, that might work as a diversion and give the former victim a chance to escape.

Any thoughts?

-Max out