Is there a way to fast lvl in single player? I thought after you killed diablo, you'd be able to play nightamre and hell. It doesn't seem so. If nothing else anyone know how to cheat and change your characters lvl? Thank you

You can start a new game with the old character, that's all. No additional difficulty levels or anything.

You can't normally play Nightmare and Hell in singleplayer however there is a glitch that allows you to.

You first need to level a character to level 20/30 respectively in LAN or on Battle.net. Create a game in Nightmare/Hell and then exit back to menu and enter a singleplayer game. The difficulty will be whatever you last played in MP.

Note you will need to do this again each time you open the game, but it only takes a few seconds.

Visit an abandonware site, download the Hellfire expansion. One of the many additions it brings to the game, is adding all difficulty modes for Single Player, as well as the ability to join any, right away, from the very start (sorcerers can start in Nightmare, kill one mob by spamming Firebolt, get 2 level ups, rinse and repeat, until they are ready for Normal).

To install Hellfire correctly, just see through the sticky thread with instructions.

Have fun!