How can I fight the wall of flesh without killing the guide?

I had the same feelings when I was ready to enter Hardmode. There apparently is not a way to summon the Wall of Flesh or any other bosses from the vanilla console, but there are mods.

I used TShock, a Terraria server that lets you spawn any boss (among other things). It worked nicely, and if you want to quit using it after you beat the Wall of Flesh, that's fine - it's an external server, so there's nothing to uninstall from your game.

1. Back up your saves! Just because it's better to be safe than sorry.
2. Download TShock and start the server.
3. Join the server via multiplayer, using 127.0.0.1 as the IP and the default port.
4. Become an admin, as described in the instructions linked in step 2.
5. Enter /spawnboss wof in the in-game chat, preferably while in the Underworld.

The only way to summon the wall of flesh involves killing the town guide.

This is not a great loss, as the guide respawns quite quickly, and there is no penalty for killing an NPC.