You asked me to draw a clear line about where killing is acceptable and not an "evil act." I did. It's not my fault you don't like the line. You didn't ask me to try to "increase the good" by offering baddies the chance to have the epiphany. You asked for something more simplistic - how do I avoid evil? (Maximizing good and simply avoiding evil are two very different things

). My "line" wasn't necessarily drawn to keep a good character good, it was drawn simply to keep a neutral character from crossing into evil. Please show me where the line is internally inconsistent.
We can deal in what if's - "what if the evil person becomes good?" - but they're ultimately speculation, not cold hard fact (I presume here you mean he turns good after sparing his life, not killing someone who is now good because once upon a time he was evil).
The fact is, when he was killed, he was evil. So he never became good. The "what if" doesn't matter - because "it didn't happen."
(I am here reminded of the LotR:RotK EE easter egg...
"What if the ring WASN'T destroyed?"
"It was."
"But... what if... it wasn't?"
"It was!"
"But... what if... you know... it WASN'T?"
"It WAS!")
Yes, it's a simplistic worldview. But is completely internally consistent, no? When he was killed, he was evil. He never became good. Where's the problem?
(I have posted elsewhere that ideally, a paladin offers his quarry the chance to repent unless offering that chance is likely to lead to the harm of an innocent, but since we're not talking about "increasing good" but simply "not doing evil" here, it's somewhat off-topic.)
Allow me to answer your question about the child not quite as stated, but instead substituting the word "crime" with "evil act" (to avoid the whole, 'whether or not something is a "crime" is a function of local laws" side trek) and address it from that angle.
I reject the premise that you can be "evil" and not yet have committed an "evil act." Alignment is "who you are" - and what you do is a function of "who you are." It is in action (or deliberate inaction) that one's goodness or evilness - or neutrality (a mix of both that doesn't heavily favor one or the other) is displayed. These actions need not be the "ultimate visible act of evil" but thoughts, words and deeds in preparation for that act are also evil (e.g., if you're plotting to murder someone, the acquisition of poison, the planning process to insinuate yourself close to them, etc. are evil acts at the time of commission... they aren't neutral acts that suddenly become evil when the "ultimate act" of murder occurs).
You defined the child as "evil" in alignment, therefore he must have committed acts sufficient to classify him as "evil" in the first place - someone with an evil disposition but who doesn't ever act on it is "neutral."
Again, "alignment is what you are, and what you do reflects what you are." If you haven't committed any evil acts, you aren't "evil," you're "neutral" ... so your question is a meaningless paradox.
The answer I would give is, "if the 12-year-old is evil (then he will have committed evil acts); it is not evil to slay him." If the 12-year old has not committed evil acts (then he is not evil); it is evil to slay him. Whether or not it is evil to slay him is a function of which of the two conditions you asserted is true; both cannot be true at the same time.
I would structure it thusly using formal logic.
1 - If X is of evil alignment, then X will have committed evil acts.
2 - X is evil.
3 - X has not committed evil acts.
4 - Because X has not committed evil acts, X is not of evil alignment (Modus Tollens).
Conclusion: Line 4 and Line 2 contradict each other, therefore one of our premises (1-3) must be wrong.
If you wish to reject my premise (i.e., a necessary part of moving from "neutral" to "evil" in the first place is the prior commission of evil acts), fine... but since I hold it as a true principle unless proven otherwise, you will need to either prove otherwise or cede that we cannot continue to have a meaningful argument about the original question. I'm not telling you "disprove this!" I'm just saying if you don't agree, we're probably at a logical impasse on this one and will have to move our attentions elsewhere... and I think we may be.
The simple answer is, you don't need to have perfect knowledge. You meet a creature. If you know it to be evil (e.g., a demon), you kill it on the spot. No evil committed.
If it threatens you, you kill it. You are in defense of life and limb, regardless of its alignment, and if you kill it, you're not committing an evil act. Hungry animals, mindless constructs, etc. are likely to fall into this category even if you don't know exactly what it is.
If you are not familiar with the nature of the creature, and it is not an immediate threat, you ought to take a moment or two to parlay in order to determine its nature (problem is, most PCs don't know how to do anything but slay; parlay is all but an obscene word) - thus solving your problem of lack of knowledge. If it attacks when you parlay, go back to "it threatens you" above.
Does this leave you open to treachery? Of course... but that's one of the "costs" of being good (in the same way that the old "



-for-tat" solution to the prisoner's dilemma game is that you always get bitten first, but this is offset by higher potential gains from cooperation and taking the chance that you can get those gains - but that's getting off-topic).
(Note that it's quite possible to have two good-aligned people fight it out and have one of the slay the other and have that slaying NOT be an evil act. For instance, maybe there is a good-aligned orc among the evil orcs in the old keep. The players break into the old keep and start slaughtering evil orcs left and right (not an evil act). The good-aligned orc comes to their defense, as there is no lawful (different alignment axis, remember) reason for the slaughter and kills a paladin PC. The orc hasn't committed an evil act (defense of life and limb). Another good-aligned PC then slays the good-aligned orc. Defense of life and limb, not an evil act. You now have two good-aligned characters who have slain other good-aligned characters... and no evil acts committed.)
--The Sigil
EDIT: Apparently Eric's grandmother doesn't like the "
this-for-tat" strategy. Oops!