The original post really highlights why it's a good idea why there aren't rule mechanics to enforce and punish alignment. Often, actions can be perceived very differently by 2 different people. The implications of the action could largely be either subjective, or dependent on the style of your gameworld.
Regardless, I don't see this as evil. In
3E, this *might* be considered "Chaotic Good", or "Neutral Good". I believe "Good" covers both of these alignments now in
4E. I don't think it's Lawful Good though.
Part of the problem is also you have to bring up the debate of whether an action is inherently good or evil, or whether the intention of that act matters. That belief varies from person to person, and should be the debate of philosophers, not players. ;-)
Also consider, that while you may have automatically thought it was evil, some DM's would have considered it evil to let them go. Which brings us back to alignment being a subject of debate and subjective. In fact, in a recent thread, someone posted that as DM that his players had stopped a villain, interrogated him, and when faced with the choice between letting him go or killing him on the spot, they spared him... the DM punished them by having the villain go on a killing spree through various villages and only leaving a handful of survivors to relay the message that he he was set free by the party adventurers.
I suppose that example really leads to the answer of, it depends... what style of game, and morality do you want to encourage in your campaign?
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