A peaceful hobbit took the reflection amulet from the top of Sokoban. I know the alignment penalty for killing it is small, but I'd prefer to avoid it if possible. How can I get my amulet and avoid the penalty for killing a peaceful creature?
The penalty for killing it is actually pretty small, only one point. Which is regained easily by killing normal monsters. I would not bother to much thinking about it.
Funny detail, killing a peaceful has a -1 alignment penalty. Killing a hobbit has a +6 alignment bonus. So net you gain points. With a big if. You only get the bonus if the hobbit is hostile. So you first need to make the hobbit hostile and not kill it in one hit.
Murdering peaceful humans does have a large penalty. You lose two points of luck and telepathy. Unless you are chaotic, which has no penalty.
If you are doing it for moral reasons, tricking the game into killing it is still breaking your moral code. Bad hacker! No ascension for you.
If this doesn't bother you, you might try using pets, or Traps. Magic doesn't work (and does not give a warning). Perhaps polymorph system shock might work. But not sure on that.
Peaceful monsters don't pick up objects, so the hobbit didn't get the amulet from its Elbereth spot. Your pet may have picked it up and dropped it elsewhere. Another monster may have picked it up afterwards, or it may not: check every square of the level. Also, an Elbereth-ignoring monster (mostly
@, in particular werecritter in human form and elves) may have picked it up and even worn it. It's even possible (albeit unlikely) that a monster picked it up then fell through to another level. The amulet may also have been eaten by a rock mole, rust monster or xorn.
To answer the titular question (even though the hobbit probably doesn't have the amulet), a nymph or monkey can steal an object from a monster without risking killing it. A monkey basically has no chance against anything; a nymph against a hobbit is a fair proposition. A nymph will wear the amulet. You can use conflict to make a hostile nymph try (beware that it may get killed instead). You can tame a nymph; it may kill the hobbit. You can polymorph into a nymph and attack the hobbit; this automatically triggers a steal attack in addition to a hit, and I think the hit won't kill the hobbit if you're unarmed and have no bonus (but I'm not sure about how much damage an unarmed polymorphed player causes).
Of course, the easiest thing would be to kill the hobbit. If you really don't want to do it to maintain your out-of-game alignment (the in-game penalty is too small to consider), you could let your pet do it.