I killed a PC, I liked it.

[NYYanks6083]

PC deaths are part of the game. Honestly as long as you weren't going out of your way to kill that particular PC, or the villains were acting illogically/against their nature just to make sure that PC died, resurrection exists for a reason. Based on the situation described, I don't think you did anything that would be considered bad DMing.

The game is not Player VS DM!

This is very true.

...but...

...the DM makes the monsters and NPCs knowing that the players are going to kill them. The DM might fall in love with a particular monster or NPC. The DM might devote large portions of his or her life creating a complex and entertaining story, a dangerous dungeon, a great villain...The DM might spend hours or days or even weeks designing the perfect world of adventure for the players...

...only to watch the players sweep aside their beloved monster, bypass the trap, utterly undermine the strength of charater possessed by the villain, and burn the world to cinders. And laugh while doing it.

I do and don't sympathize. I've made some pretty epic bosses (literally, the last 3.5 campaign we ran went from level 9-19, and the final four bosses were all lv 22+, safe to say they weren't put together in 10 minutes). They fought the good fight and, predictably, lost it in the end. Was is sad to see them go? A little, but the battles against them were exciting and challenging for the PCs and fun to run for me ("He turned into a what now? with how many heads??" ) I also enjoy making bosses, and have tons of ideas for new ones, so to me it was just an opportunity to make a new, even more insane villian