OK.. Let's try this one..
You come across a woman being attacked at knife point, and it's obvious that she is about to be raped.. (let's say in a park or something, not inside a house). SO, you know he is not defending himself, but about to perform a terrible crime.
Do you just walk away?
Walk away and call 911, knowing that the deed will be done prior to the police arriving?
Shoot the bastard?
OR.. coming to the the Original Posters main concern "Financial Burden" Do you draw the attackers attention (by yelling at him, shoving him, etc), something to break off the tunnel vision of his attack and thus stopping the crime. It is unlikely he is going to continue knowing that you are standing there watching, especially since you already have your phone in your hand calling 911 right?? So, he is likely to do one of two things..
One, run away
Two, turn aggressively toward you - Now.. IF YOU should need to stop the threat, you are NOT protecting a third party, you are protecting yourself. You will run through your usual list of rules you have in your head before you pull the trigger to insure your life was threatened, you have 911 on the phone, they will know the guy was in the middle of a crime & hear you telling the guy to stop before the shots are fired. You have at least the woman that was being attacked as a witness..
So, I agree with the original poster in general; However, there are circumstances that morally you should intervene... The important thing in these rare situations IMO is to break off the threat to the 3rd party and if the threat is then drawn towards you, IF YOU MUST, then you eliminate the threat. You are no longer protecting a 3rd party, but protecting yourself..