Charges for voluntary manslaughter?

You are allowed to use appropriate force to defend either yourself or someone else that is threatened. If you or the other person is being attacked with deadly force, you can use deadly force in response. You are not "retaliating" (that would be going after someone when he is no longer a threat).

There are some exceptions--you generally can't use deadly force if you were the initial aggressor, most places you have a duty to retreat if you can with safety (which wouldn't apply if you were stepping in to aid someone else being attacked), and it can be risky to come to the aid of another if you aren't sure who attacked first. You also can't use more force than necessary to stop the attack--i.e., if the guy drops the knife immediately and runs away you can't go after him to kill him.

Manslaughter is normally defined as homicide where the malice required for murder is lacking as upon a sudden quarrel or fight. It can be voluntary, where an intent to kill is evidenced, or involuntary where the death is accidental but caused by gross negligence. Defense of another is normally permitted, so that if the other person would have been acquitted upon a theory of self defense, so would the rescuer.

The Person in that Situation needs a good lawyer.