Okay, here is my two cents worth (from the cop). Everyone has been telling you, you did the right thing. They are 100 per cent correct. Closeup probably described it best so far. No life is worth money...you can always get more money...and that is yet another reason we have insurance. You can't collect if you died trying to be a super hero.
When we/cops are off duty we are trained to be the very best witness we can be. Even though I am always packing my 45 cal. Sig-Sauer P220 with handcuffs and an extra magazine when I am off duty...I lack all of the protection my uniform and duty equipment provide. We try to get involved when the tables have turned to our advantage. Like... my position affords me cover from bullets, I have time to draw my weapon, innocents are clear of harm, you know where all the bad guys are... Do you get the point? There are tons of variable that have to be evaluated before engaging someone intent on violence.
And the most important goal of cops...and it is those people who interfere with that goal that cause some of the violent reactions you see from cops (which the news media never explain). You should make this your personal number 1 goal as well.
** I am going home to my family at the end of my shift. **
Try to image what could have happen if you had tried to disarm that SOB in the wrong direction. You could have a bunch of grieving people instead of just scared people.