Personally, I would have felt bad. Killing a PC because of the cowardly actions of his PC companions isn't really fair. But it is the other characters that made it unfair - not the DM. I would have done it (killed him) because I knew it had to be done.
My group had a PC die last session because no-one bothered to render aid while he was down and dying. After the fight, I pointed out that there had been many, many occasions when PCs had made it to -8 or -9 before being helped, and that if they continued to ignore downed PCs, more were going to die.
They stopped, had a look at how they had been behaving, and realised I was right.
In future, I think they will be a lot more concerned about stabilising each other.
In your campaign, I think the PCs should be giving some serious thought to the fact that the dead character could easily have been any one of them, and that they are, in part at least, responsible for the death.
If you want to be mean, have the PC come back as a ghost, seeking revenge on his cowardly companions.
