Arravis, here's another bit from the SRD:
Good characters and creatures protect innocent life. Evil characters and creatures debase or destroy innocent life, whether for fun or profit.
I would submit that the paladin's wife (and without getting into a possible religious flamefest here, I'll say possibly also the unborn child) are examples of "innocent life."
She/They has/have been assaulted, and need protection is needed.
Protection certainly can mean doing what is required to preclude further assaults.
The halfling was involved in the initial assault and has not indicated he would change his tune (therefore he is by definition not an innocent life, and destroying him is not an evil act), ergo he is a threat to be involved in another assault.
Removing that threat of future assaults by killing the halfling can therefore be easily construed as a
good act.
As to law vs. chaos,
Lawful characters tell the truth, keep their word, respect authority, honor tradition, and judge those who fall short of their duties. Chaotic characters follow their consciences, resent being told what to do, favor new ideas over tradition, and do what they promise if they feel like it.
“Law” implies honor, trustworthiness, obedience to authority, and reliability. On the downside, lawfulness can include close-mindedness, reactionary adherence to tradition, judgmentalness, and a lack of adaptability. Those who consciously promote lawfulness say that only lawful behavior creates a society in which people can depend on each other and make the right decisions in full confidence that others will act as they should.
“Chaos” implies freedom, adaptability, and flexibility. On the downside, chaos can include recklessness, resentment toward legitimate authority, arbitrary actions, and irresponsibility. Those who promote chaotic behavior say that only unfettered personal freedom allows people to express themselves fully and lets society benefit from the potential that its individuals have within them.
Without knowing the circumstances of the local law, it is unclear as to whether the paladin is being obedient to authority. In some areas, he might be judge, jury, and executioner by virtue of being a paladin. In other areas, he might be judge, jury, and/or executioner by virtue of being kin to the one who was wronged (e.g., in some cultures, the husband, father, and/or brothers of a woman wronged are legally allowed - and in fact obligated - to execute punishment on those who wronged her). In yet other areas, he might need to submit to local law enforcement. As (at least as of the first page of the thread, haven't read the entirety yet) we don't have the social context, it is unclear as to whether the paladin is being disobedient to authority (if punishment is in the hands of local government) or exceptionally obedient to authority (if punishment is in the hands of kin).
One could argue that he was following his conscience, and that is a Chaotic act, but if one's conscience prompts one to follow the law (see above), I'm not sure if you can call it truly chaotic.
I can't see how this attack was evil by the RAW, given the information we have on the halfling (part of the assault). It is unclear as to whether it is chaotic by the RAW.
--The Sigil