Murder: Law. the killing of another human being under conditions specifically covered in law. In the U.S., special statutory definitions include murder committed with malice aforethought, characterized by deliberation or premeditation or occurring during the commission of another serious crime, as robbery or arson (first-degree murder), and murder by intent but without deliberation or premeditation (second-degree murder).
Kill: To deprive of life in any manner; cause the death of; slay.
1) the paladin neither killed nor murdered the ghost. The ghost had no life to deprive, and thus was not killed, and murder is a legal term for killing.
2)
Rejuvenation means that the ghost is coming back;
all ghosts have it. Hard to say the ghost was "killed" when he'll be back in a week.
3) These monks, who have been around a ghost (who posesses the "Rejuvenation" ability) for so long choose not to say, "Oh, he'll be back, let me tell you why you should not have done that..." and instead attack. The monks bring a paladin into their monastery, and without preparing him or attempting to accomodate his status as an anti-undead warrior, march a ghost in front of him. This is pretty bloody careless. (
This sways the Evil/Not Evil debate in no way, but it is an observation I thought I'd bring up.)
4) When attacked, it is not evil to protect yourself.
If there was an evil deed here, it is destroying the ghost, and
not protecting himself from hostile monks.
This paladin believes (because he's been taught it by the church) that undead are anaethma to all things Good. If you have a relativistic morality system (though I disagree with them, some folks do have them in their games) then this is all the vindication he needs. Not Evil.
If the game has an Absolute Morality, then it is up to the DM to determine the Truth: is an undead's
existance [Evil]? This is not a question of if the undead character is evil, but rather if the presence of a Negative Energy Powered "life" itself constitutes something [Evil]. If yes, then the paladin is in no way guilty of having committed an evil act. If no, only then
might the paladin have committed an evil act. Only the campaign's DM can answer this question, and further debate without the answer is fairly well spurious.
NOTE: An undead character may be something other than [Evil] while at the same time the very fact of his existance be [Evil]. This is reconciles Ghosts of any alignment with the fact that they all detect as
Evil for spells like
Detect Evil.