The war with the Outcasts is going ever since the CR settled in the Taus. As the CRs origin lies within a UN mission to space, there is a very strong dislike for slavery and the overall way the Outcast society works. The 72th is a bit different, which is why they were granted a chance.
As the CR-OC war is going on for many years now, the destruction of every potentially war critical ship (yes, this also includes unarmed battleships, if you are a hospital then use something else) has a rather high priority. That this individual situation could have been handled betters is correct, but the outcome would not have changed much (except if the person in question would have surrendered the ship to the CR and the people on board would have been freed/held as hostages depending on where they came from, which is rather tricky from both a gameplay and a RP-perspective and usually not possible within the frames we have to work with).
In the eyes of the CR there is no real difference on whom we hate more. The war against the Outcasts is running longer and the reasons are still there, while the war against Gallia was shorter but caused us more losses, which have particulary been undone by now. Though if we were to chose whom to kill first, then slavers might usually be the first ones to die.