It's hard to describe via the title, so I'll post some examples.
killed by 
"Diana... why..."
killed by 
"Fat... Hands..."
killed by 
"N-Not... like... this..."
killed by 
"Traitor..."
killed by 
"Y-You... varmint..."
NOTE! Try to keep the death animation in mind.
killed by 
bong, bong, tingl-ling-a-ling! RRRROWRRRRR! BOOOONG! SKREEEEEEE!
-gasp- such language!
killed by anyone
''SHURIMA SHURIMA SHURIMA SHURIMA''
One of them kills the other.
"You're... Adopted!"
killed by 
"Better to keep your reputation as a hero than to befriend a filthy rat like me"
killed by 
"You think killing me will save your wife from eternal damnation?...ha...ha..."
killed by 
"Mundo only wanted to ask you out..."
killed by
"she's a killer....queen..."
killed by /
/ /
"Betrayers...must perish"
killed by 
"I always knew you would suck me dry, but not in....this...manner..."
killed by
"It seems like the student has surpassed the master"
killed by 
"But....i'm the cutest"
Killed by 
"Tibbers... is that you?
killed by 
: "Hold my hand once more brother..."
killed by a void champion: [angry sounding chimes]
killed by 
: No... how...
Killed by 
" I guess you could say I died Hard"
Killed by 
" One Bird killed my 2 stones"
Killed by anyone
"How did they know?" (when you kill a clone) "Look behind you (laughs)"
Killed by 
"Should have expected the banana"
Killed by 
(plays walk the plank song)
Why they always gotta stutter while trying to say the first word xD? I do like the thought of special dialogue though.
killed by 
"You...were always...Father's favorite."
killed by
"You...Street rat."
killed by
"Heeheehee who would have thought I'd hesitate."
Killing his clone.
"Oops you just jinxed it. Mrweeheehee"
killed by
"But you don't have a doctor's note..." or "Can't wait till I see you again."
killed by
"Someone I don't want to run."
killed by
"Out foxed by a wolf."
killed by 
"Finally a worthy death for a warrior."
killed by
"No fair, you have no blood..."
killed by
B-but revenge runs deeper then blood.
killed by
I regret not being able to give you the death you deserve.
killed by
You..can...milk..those....
Killed By 
"I liked you better when you stank!"
Killed By
" How...could..I..live..smelling...like..that......"
Killed By
"You SON OF A BIIIIIIIIIIIIIIIIIIIIII-"
killed by 
"I CANT BELIEVE I WAS BEATEN BY A NEEEEEEEEEE.......RDDD" Splat
killed by
"You mechanical bru-!"
killed by [ANY ELECTRICAL CHAMPION]
"ERROR 404!" "BLITZCRANK 98 HAS CRASHED!"
killed by 
"You..creeep..."
killed by
"I JUST WANTED TO LEARN"
killed by [REVERSE ]
"WHY WONT YOU LOVE ME!!?"
timeouted by 
Hic Hic Hehhh.... hic
whacked by
"I knew I shouldnt of let you have this..."
[ARCHDUKE] Killed By
"Rude, buffoon! My coat!"
Killed By
"NASUS! YOU'RE AN ASSHOLE!!!"
killed by 
"Ha ha ha... now... you will never see her... again..." fizzles intothe void
done in by
"Warwick... why are you doing this?"
Alternatively
"STUPID DOG"
