Without any spoilers.

I got tired of the Guide, so I trapped him in a room with a slime and he died.

Now I read, that his death triggers some hardmode. Did I doom myself (I started just recently) or there is some additional trigger required?

Ok, thanks, I am glad I haven't screwed anything yet haha, wonderful game.

Feel free to kill anyone, and they will come back (with a different name), with the possible exception of the Party Girl, though I may be wrong about that. If you manage to kill one of the traveling salesmen you get their hat; it doesn't give you any stats but it looks really cool. :P

To summon the boss to trigger hardmode you have to throw a guide voodoo doll into lava

If the Guide has a certain name, then he drops a green cap.

He always drops it. Also, if you aren't aware of it, throwing the clothier voodoo doll in lava summons Skeletron (and kills the clothier in the meantime).

And if you are not aware of it, equipping the clothier voodoo doll allows your attacks to hit the Clothier and finally kill him. But Skeletron will only be awaken when you kill him during nighttime.

btw u have to throw the guide doll in the underground lava...so dont be afraid of killing him as often as u want...some of your npcs got some unique drops too...like the paintball-gun from the painter, the scissor from the hair stylist or the luigi cap from the guide....