It happened on our last session. The party was facing a boss, a defiler who had just finished a ritual to transform him in some kind of demon draconish sorcerer, and they were winning by just a little (two of six were down and making death saves).
The boss was with just 30 hit points when he decided to make the run. He managed to escape through the door and run through a corridor leading further in the dungeon. When he ran out, the party didn't chase him, they choose to stay together and heal the dying and the poor child used in the ritual (they had to pass through a skill challenge to save the child's life). I should have said the majority of the party, the barbarian decided to run after him and end this once and for all.
The defiler seeing the barbarian chasing him, alone, turned back and fired his necrotic bolts at him. The barbarian dropped and the boss ran away. At that point the group was in the middle of the skill challenge to save the child's life, trying to remove all the necrotic and demonic energy around the kid. They did heard the cry of the barbarian when he was hit and fell, but they had to choose, they would either run to save their companion or the child. They trusted the barbarian would not fail 3 death saves too quick and would be able to save him after the kid, but they were unlucky.
The child was saved (although it cost the infection of a demonic force inside one of the characters), but the barbarian died, he failed 3 saves in a row, he was dead. I killed a character, and it wasn't that difficult as people say it is in 4th edition.
At that time I felt like I had won something, I was kinda in the villain mode, so I was excited he managed to escape and take someone out. But a few minutes later I was worried, I just killed one of my player's character. Is he upset? Will he be willing to make another one? Or will he be hating this game now?