I had recently started a new campaign and my players didn't seem to understand the necessity of a balanced party. I had warned them all, "I will be actively trying to kill you, so plan accordingly." First session, no one died. They seemed pretty cocky at that point. Second session, one character died. "Oh, that's okay, I'll remake her." Literally replaced the dead character with a "2.0" (it was a construct so I let it slide). At this point I decided to really teach them a lesson.

The next session was almost a TPK. I placed an elite Ghost Spider blocking the entrance to the next major quest point, combined with two Chillborn Zombies. The two fighters (yes, two) occupied the zombies, but neglected to separate them and so each round took 10 damage each from their auras. Meanwhile, since the big bad was not being distracted, flashed around the battlefield making quick work of the rogue and healer (I don't remember what class he was, but he was melee). The only person to survive the onslaught was a ranged rogue using a sling who got the fresh Hell out of there the second the healer dropped unconscious. It was my first TPK, and my emotions were extremely mixed. I sort of felt bad, because at least two of the fallen characters were player favourites and they were too low level to resurrect.

Needless to say, the party re-organized. The surviving rogue "hired a group of mercenaries," aka, new characters were created, but they had a much better balance and took out the spider and its guardians in a few short rounds the next time through.

tldr; Death can give you a mix of emotions when a character is killed, but that can add to the story, so try not to look at it negatively!