If you hit someone with a wrench it's called Assault With a Deadly Weapon. It's legally not viewed much different than attempted murder. In fact lots and lots of shootings result in ADW charges (name varies but the law is the same) not attempted murder.
Before you go all ninja on someone you'd better be damned sure that you are doing the right thing, and your "gut" is the last thing on earth you'd better be using to make the decision to get involved.
Everyone likes to talk about their "ideal" situation. In fact, that arises very rarely. What you're much more likely to find is a situation that is anything but clear as to who is the aggressor and who is the victim and how serious the situation is. You're drastically more likely to face a situation where using deadly force would be illegal, particularly in Mass where the use of deadly force threshold is very high.
No one here is going to stand by and do nothing while someone stomps a little old lady. But in fact it's pretty damned unlikely you're ever going to see that happen. You wanna bust a cap in someone's ass for snatching a purse? Have fun in Walpole. Are you going to go all Rambo and chase the guy down? I'll notify your kids what a hero you were as you died for $200.
If someone sticks a knife in my ribs all of a sudden and demands my wallet he's going to get it. when it's prudent or absolutely necessary to protect myself at the risk of getting stabbed, I'll pull my piece and that will be that (Or I'll be slower than him and die trying).
911 is your friend. Call the cops, be a good witness, get plate numbers, descriptions etc. and as a very last resort, intervene if you think someone is about to die or be grievously injured.