Might immediate retaliation or reaction inhibit future bad kamma? If someone hits you and you even the score, purely to HELP them avoid future bad kamma           (like if your intention is purely that... you want to neutralize their bad kamma... not an excuse, no anger at all, general concern):


Would it neutralize the bad kamma for the other individual?
Despite necessary force to subdue the assailant and keep things even steven... would it still be good kamma based on intention?
For the first question the answer is NO. In AN 5.57 we can see that Buddha has used this phrase,


  I am my own kamma, I am a heir to my kamma, I am born [in this life]
  from my kamma, I am the kinsman of my kamma, I am protected by my
  kamma. Whatever kamma·s I shall do, kalyāṇa·s or pāpaka·s, I shall
  become their heir.


So you can't neutralize someone else's bad deeds by your actions. He will eventually reap the results of his actions when all the conditions fall into place. I think it's easy to understand this if we consider kamma as a natural law like gravity. 

There are four types of kammas with respect to the time of taking effect in abhidhamma.


  
  Immediately effective kamma (Dhitta dhamma vedaniya) - in the present lifetime
  Subsequently effective kamma (Aparapariya vedaniya) - in the immediately following lifetime
  Indefinitely effective kamma (Upapajja vedaniya) - in lifetimes two or more in the future
  Defunct (Ahosi) - kamma whose effects have ripened already
  


If I consider your example of hitting back when someone hits you, we can categorize this as a immediately effective kamma for the person you're hitting back. But we can't still say the power of his initial kamma has weakened. In another life, in several other lives or even in this life he may reap the results of this. If the conditions didn't fall the kamma may never function as well. 

For the second question the answer is also NO. You may not have have anger at the point you hit him, but your intention is filled with delusion. You are purposefully hurting someone with a wrong view in your mind.