One thing that is very important in hell is fire resistance. The fire traps in Gehennom are particularly nasty because, unlike normal fire traps, they decrease your maximum HP unless you're resistant. You should also fireproof your armor (by reading a scroll of enchant armor while confused for example) and put all items that could burn or boil into a bag, so they won't be damaged or destroyed.
As to the level drainers: The best defense against level drain attacks is magic cancellation. If you have an item which offers level 3 magic cancellation, this will cause drain attacks to fail in most cases, making level drainers much more harmless.
For fighters it's also a good idea to have a silver saber as those are especially effective against demons and the undead (well, most undead - zombies don't care, but zombies aren't really dangerous at this stage anyway).
Also note that you can eat wraith corpses to gain levels.
General things you should do before you enter the end game:
Get your naked AC down to 2 or lower with divine protection (you can get it down to 0 quite easily via donations).
Have magic resistance and reflection.
Be fast (or preferably very fast)
Make sure all of your stats (except charisma) are close to or at their maximum
Enchant most of your armor to +4 or +5.
If you're a fighter, enchant your weapon to +6 or +7.
If you're a caster make sure that all your important spells are recent in your memory or you have the spellbook in your bag of holding. You do not want to forget magic missile while battling The Wizard or Death.
Make sure you have a means to detect portals (not important in Hell, but once you leave the dungeons, that's gonna come in handy and you do not want to have to go searching for gold detection scrolls after you have woken The Wizard).
You should also have a reliable way to cure sickness (blessed unicorn horn or cure sickness spell) as there will be a couple of enemies who can make you terminally ill.
The most important thing to note is that identifying items in one game only helps you in that game. If ruby rings were rings of invisibility last game, but you died and started a new one, you know NOTHING about them. The exceptions are the trivial ones: potions of water and blank scrolls/spellbooks.
The second most important thing about identifying items is that each item can be blessed, uncursed, or cursed. If your role is Priest, then the blessed/uncursed/cursed status is automatically revealed to you for all items. If you are of any other role, then you need to utilize alternative methods to determine the status. The available options are:
- You can figure out which items are which by dropping them on an altar (as long as you are not blind, so that you can see the flashes of light).
- Reading a scroll of identify will also point out the status of the item.
- As mentioned in Durathor's answer, pets will avoid walking on cursed items that are on the ground, only moving "reluctantly" if they ever stand on it. You can use this from the start to avoid an early demise from using cursed equipment.
Blessed items and uncursed items are much safer to use than cursed ones. Don't drink any cursed potions, read cursed scrolls, or put on any cursed rings/armor unless you know exactly what you are doing.
Rings and armour
Conversely, if you have rings or armor that you know is not cursed, you can put it on and take it off right afterwards. For certain types of equipment (ring of invisibility, gauntlets of dexterity, boots of levitation, etc) this may identify it immediately. Others will have clear effects, allowing you to #name them yourself. (Ring of conflict, gloves of fumbling, etc) This will also tell you how heavily enchanted a piece of armor is.
Note that certain types of rare headgear can still cause problems, even if they are not cursed. The helm of opposite alignment will lose you all your divine protection, and will block you from doing the quest. Anything that looks like a "conical hat" should be avoided unless you are a wizard, in which case you may want to wear the expensive type.
Potions
Potions provide you with some interesting options.
- Clear potions are always water. However, they may be blessed (holy water) or cursed (unholy water), which have important uses. You can find out what kind of water you have by dropping the potion on an altar (they won't break, promise).
- You can try #dipping a unicorn horn into a potion. Most bad potions will be neutralized this way. If you dip a unicorn horn into a brown potion, and the brown potion clears/changes color, don't drink any brown potions.
However, just because the potion did not change, does not mean it is safe to drink. Unicorn horns don't detect everything. (sleeping and acid, for example). Also, a potion of polymorph will cause you to lose your unicorn horn as it turns into something else. This can be prevented by dipping any other item into the potion first.
- If you find a food shop, you can try dropping your potions there. Anything that the shopkeeper offers to buy from you is a "food potion." In practice, that means water, fruit juice, or booze.
- Monsters will drink helpful potions (speed, healing, invisibility) and throw harmful potions (paralysis, blindness, acid) at you.
When in doubt, wait for this to happen instead of drinking random potions. The penalties are much less severe, especially if the potion was cursed.
- If a potion was dropped by a nymph, and it's not one she stole from you, chances are that it's object detection.
- The potion of oil is unique in that you can apply it to light it on fire like a lamp. No other potion is a valid target to apply.
Scrolls
For scrolls, I typically end up relying on price. As with all items, more expensive scrolls do 'bigger' things. Some specifics on scrolls:
- First level of Sokoban always has two scrolls of earth in the same spot.
- If you find a scroll in a one-square "closet", it will be a scroll of teleportation.
- Scrolls of identify are really cheap. Cheaper than blank scrolls. This makes them pretty easy to identify.
For more information, including a very detailed price ID guide, see http://www.steelypips.org/nethack/id_faq.html
Best Answer
As you guessed, killing your pet either directly or via some indirect actions (such as displacing it into a body of water) can potentially anger your god or affect your stats. However, yes, there are a few ways that you can separate yourself from your pet, with no negative consequences:
All that said, you may want to reconsider. If you let them get enough XP to level up properly, even your starting pets can become incredibly powerful, and will be very helpful to you in combat during the earlier parts of the game. If you're getting annoyed with your pet's movement patterns, using a leash (if you can find one) will keep your pet closer to you, and keep it from moving quite so erratically. Additionally, later into the game there are some incredibly powerful dungeon monsters that you can tame to be your pet as well, which can be a huge benefit.