sorry to hear you are going through all this, I am not a guy, but can see clearly she needs help to curb her anger. she is probably feeling insecure at the moment, having been through abusive relationships in the past makes women 'snap' much faster than they probably should, it is a defence mechanism. saying that it can be reversed, it takes time, stability, a caring man and therapy to get over this. she needs to realise you are the protector in the relationship, that you will protect her. also that there are other ways of dealing with abusive people than using violance. words are often more harmful than fists (not as satisfying but hurt longer) also there are other actions she could use e.g. apply for restraining orders, avoid the abuser, turn and walk away etc, all are better than fists. she has probably only known violance as a way to deal with abusive situations, so lead by example, by seing you handle situstions in a non violant way will encourage her to also. abusers will get bored if you do not react to their abuse, thats because abusers thrive on reaction, by ignoring their abuse ok they will try harder at first, but eventually give in and move on to someone who does give them the reaction they crave.
try having a chat with her, explain that you love her and want to be with her as a married couple, explain that in marriage there are certain things that a man feels are his responsibility, those being blaa blaa blaa and protecting the family including his wife. from there you can go on to explain how seing her decking a man makes you feel inadequate as you see it as your role to deal with those situations, you could then go on to give her suggestions of things she could do e.g walk away and phone you rather than deck the guy or phone the cops. Always end on a reasurance that you love her and will do anything to protect her.
give it a go, it can't hurt, and may be just what she needs to hear. she may not be aware of how it makes you feel or of the options available to her in how to deal with abusers.
You seem to be a one in a million kind of guy, most would run away from this situation, it shows how much you care for this girl that you want to stick around warts and all.
good luck
take care x