How to get an item from a peaceful monster without the murder penalty

First off, no matter the corpse, never eat unless you just made that corpse or you have a method of preservation (tinning, icebox). Rotten corpses cause food poisoning (unless you're a fungus or a ghoul), and it doesn't take very long for a corpse to rot. And there isn't a visual indication of rotting, either, so the safe bet is to never eat something you didn't just kill. Most undead corpses are automatically rotten, as well. (See below for a more technical explanation.)

If the monster has poison or acid attacks, or is made of poison or acid, then there's a very high chance that you'll be poisoned or take acid damage. Both of these can be fatal, the former even instantly so, but poison and acid resistance will protect against this. Note that most poisonous corpses will chance to give you poison resistance, so this can be something to risk if you have enough health. Other elemental effects like fire, cold, and electricity are traditionally safe.

If the monster causes you to be stunned, hallucinating, or confused by natural attacks (as opposed to casting a spell), then it is highly likely you will be tripping after you eat its corpse. Some of these are very obvious. Actually, most creatures with negative effects like petrification will also carry over that effect with their corpse.

While it won't directly kill you, the corpses of teleporting creatures like imps, nymphs, tengu, and leprechauns will often give you teleportitis. Note that tengu do have a chance of giving you teleport control, so it's a good idea to start eating those when you do get stuck with teleportitis.

Unless you are of the Caveman profession or the Orc race, you are not a cannibal. This means that eating a member of your own race will give you a penalty in the form of the "Aggravate Monster" intrinsic (which is as bad as it sounds) and a penalty to your Luck. This penalty will also show up for non-Caveman, non-Orcs when eating any domestic dogs and cats (the line of creatures you can get as a starting pet, generally). Horses are fine to eat.

For specific exceptions, off the top of my head: kobolds are always poisonous but never grant resistant. Bats cause stunning. Wraiths are safe to eat despite being undead. All adult Dragons are safe to eat, as well as most babies, but baby green dragons are poisonous unlike their adult counterpart. Lichen and Lizards do not rot over time and carry no negative effects that some other members of the same creature class might, so they make excellent backup food.

The rotting process is a little bit complicated and based on luck more than anything. Every 10 to 29 turns, a corpse gains a degree of rottenness. For a blessed corpse, 2 degrees are subtracted, while for a cursed one 2 degrees are added.

When it reaches a rottenness of 4, it is considered old. When you eat an old corpse, there's a 20% chance you will lose 1-8 hp, and a 1/7 chance of it being rotten (as in rotten food, not deadly.) When it reaches a rottenness of 6, it will give you fatal food poisoning, killing you in 11-19 turns.

So, put more simply, a corpse is definitely safe to eat in all cases for 32 turns, and it can cause food poisoning after a minimum of 48. Everything beyond that is just random chance.

Undead corpses start out with an age of 100 turns, which is why they are almost always already rotten.

Attributions for information provided within comments to:

• WillfulWizard for reminding about teleporting monsters.
• ire_and_curses for corrections on non-rotting lizards and poisonous baby green dragons.
• Kaestur Hakarl, who investigated and discovered the safe measures for cannibalism.

I've finished my game for the wizard so I can outline the strategy which was working for me. I don't know what parts you know exactly, so I will tell you everything I consider important.

First of all, dump your quarterstaff and try to use daggers as your main weapons. They can be not only wielded, but also thrown, and if you become skilled in using daggers, you will be able to throw several daggers at once. Ten daggers or so is a good number to start with. Also I'd find an armor which doesn't hinder spellcasting (like studded leather armor) and try to enchant the cloak. Obviously enough, you should try to learn as many spells as possible and get all the resistances you can.

I advise not to go into the Gnomish Mines when you find the entrance. You can reach the Minetown easily, but you won't be able to reach Mines End yet and get out safely. So instead of going to the Mines, go to the Oracle and then to Sokoban. Sokoban should train your strength because you will move boulders a lot, which is helpful, and also you will get some gold and a gift at the end, which is, as you probably know, either bag of holding or the amulet of reflection.

After Sokoban you should go to Minetown and try to reach Mines End. Minetown has an altar and if you're lucky, it will be your god's altar. If that's the case, you should offer corpses of recently killed monsters to your god until you get Magicbane. This is a magical dagger, which is extremely helpful in combat.

Mines End will give you a luckstone (and also some gems). You have to fight a lot of monsters there though, so stand on the stairs and use Elbereth to keep them off. If you have Magicbane, engraving done with it will be harder to erase.

If you have survived Mines End, you can return from the Mines to the main branch and safely descend until you come to the level with the quest portal. Don't worry, if you haven't reached level 14, the first quest level usually has Wraiths whose corpses raise your level when eaten. If you still need to gain levels, descend lower in the main dungeon. Usually I don't linger in the level after I have explored all the passages. You should aquire necessary level before you reach Medusa's island.

When your level is enough, you can finish your quest. Your nemesis won't be tough if you know how to hinder his healing. After you finish him, you get your quest artifact which replenishes your power, and later on you shouldn't have any problems.

That's the basics of the wizard strategy.