Very nice ideas, people. Just to condense the ideas into easier to manage groups:
Creatures:
was killed by a former human. (Former Human)
got humilitated. (Former Human)
was burned by an imp. (Imp)
was slashed by an imp. (Imp)
was too weak to face a pariah. (Imp)
was shot by a former sergeant. (Former Sergeant)
caught buckshot. (Former Sergeant)
was bit by a demon. (Demon)
got tempted. (Demon)
was perforated by a former captain. (Former Captain)
counts holes in his flesh. (Former Captain)
was melted by former commando's plasma gun. (Former Commando)
found his peace in superior firepower. (Former Commando)
was spooked by a lost soul. (Lost Soul)
lost his soul. (Lost Soul)
was smitten by a cacodemon. (Cacodemon)
got too close to a cacodemon. (Cacodemon)
became cacodemon fodder. (Cacodemon)
was bruised by a Baron of Hell. (Baron of Hell)
was ripped open by a Baron of Hell. (Baron of Hell)
hailed the Baron of Hell. (Baron of Hell)
was splayed by a Hell Knight. (Hell Knight)
was gutted by a Hell Knight. (Hell Knight)
became dishonored. (Hell Knight)
let an arachnotron get him. (Arachnotron)
couldn't evade a revenant's fireball. (Revenant)
was punched by a revenant. (Revenant)
met Death itself. (Revenant)
was boned. (Revenant)
was squashed by a mancubus. (Mancubus)
was stomped on. (Mancubus)
was incinerated by an archvile. (Arch-Vile)
succumbed to dark arts. (Arch-Vile)
joined hells army. (Arch-Vile)
got burned from the other side. (Nightmare Imp)
met his worst nightmare. (Nightmare Cacodemon)
lost his mind. (Any Nightmare Creature)
rode a rocket. (Cyberdemon)
splattered by a cyberdemon. (Cyberdemon)
failed in the end. (Cyberdemon)
was too cunning to live. (JC)
got outsmarted. (Spidermastermind/JC)
has no body to count bullet holes from. (Spidermastermind)
stood in in the awe of the spider demon. (Spidermastermind)
found out its not a milf. (Spidermastermind)
[color=red]just found the Spider Mastermind.[/color] (We wish!)
was torn apart. (if there was more then one of enemies around)
did not see it coming. (Spectre)
wasn't just seeing things. (Nightmare Demon / Spectre)
Level Specific:
became too berserked. (Chained Court)
got chained. (Chained Court)
couldn't survive the next wave. (Hell's Arena)
didn't enjoy the show. (Hell's Arena)
found what to do in the Wall, but payed too big a price. (The Wall)
became another brick in the Wall. (The Wall)
didn't make it to the breach. (The Wall)
became the part of the City of Skulls. (City of Skulls)
got his flesh ripped off his bones. (City of Skulls)
was spidered to death. (Spider's Lair)
became webbed. (Spider's Lair)
lost his mind on arachnophobia. (Spider's Lair)
couldn't handle the carnage. (Halls of Carnage)
was too humane. (Halls of Carnage)
found that it's really hot there. (Lava Pits)
got spoiled. (Lava Pits)
choked on a stench of rot. (Mortuary)
was declared a heretic. (Unholy Cathedral)
got corrupted to death. (Unholy Cathedral)
was not worthy to wield the Spear. (Unholy Cathedral)
was excommunicated. (Unholy Cathedral)
didn't make it through. (Hell's Gate)
got buried alive. (The Vaults)
got cursed. (The Vaults)
got loaded. (Hell's Armory)
Challenge Specific:
lived by the sword, died by the sword. (AoB, killed in melee by Baron or Knight)
fell to berserkerbane. (killed in AoB by former sergeant)
was not worthy to carry the sacred artifact. (AoB, died in UC, not by AoD)
fought the ultimate battle and lost everything. (AoB, killed by Cyberdemon)
met the Reaper face to face. (AoB, killed by AoD)
was the only one remaining in town. (AoMr YAAM)
it's not he who feared pain... It was them! (AoMs YAAM)
patient hunter got his prey. (AoPt YAAM)
was fast, yet thorough. (AoH YAAM)
emptied Hell itself! (Ao100 YAAM)
kicked ass indeed. (AoSg YAAM)
cheated reality. (AoPc YAAM o.O)
proved worthy of Valhalla. (AoB YAAM HNTR)
became a warrior of legends. (AoB YAAM HMP)
became the God of War. (AoB YAAM UV)
is feared even by the Gods. (AoB YAAM N!)
Misc:
went boom. (barrel)
failed to recognize the "Biohazard" sign. (Acid barrel)
loves the stench of napalm. (Napalm barrel)
needed a light. (Fuel barrel)
get melted. (Lava)
died from dehydration. (Lava)
got overheated. (Lava)
could not walk anymore. (Acid)
has no legs. (Acid)
was lost in time and space. (Chained teleports)
gave a birth to nasty things. (Telefrag)
was gone with a blastwave. (Nuke)
forgot about the nuclear vault. (Nuke/AoRA?)
got bored with life. (Rocket jump)
tried to push the pin back in. (Rocket jump)
wasn't Tank Jr. (Rocket jump)
believed he could fly. (Rocket jump)
There were a few English syntax errors, I tried to fix those.