This happened when he was only 9:

Nelian felt that something bad is gonna happen , specially after he met stark (an pure evil guy ), so he started looking for some new , strong power , somehow he could get a hidden power of the strongest demon in the planet , once he came back , he found his parent been controlled and surrounded with strange creatures , they were freezed at first , so he got rid of these creatures pretty fast then head to his parents when..
his Dad: * moves his head * they maybe controlled your mother .... * looks at him surprizingly * but I still with my full mind want to destroy this damn planet!
Nelian: what! * steps back *
Dad: have you thought that I'm the lovely father you always knew ? * steps on * heh! fool junk!
Nelian: no way! ... you've got to be kidding me! * steps back * wake up you bastard! * jumps on his father *
Dad: *catches Nill * now wake up from what? the ultimate power stark just gave me? or the fun of destroying planets?
Nill's Mom : * moves too , walks to him * you better say goodbye o this planet for ever! * starts generating some power ball *
Dad: * throws Nelian , then joins his mom generating the power till the ball became very huge that almost destroyed the house *
Nelian: stoooooooooop! * shouts! *
Dad: too late! * throws the ball *
Nelian: * trys to stop the attack * noo! * the attack pushes him back * no! I must ... stop it! .. or it will destroy the planet! * the attack becomes harder and harder , and his parents are increasing it , he's pushed back and cannot stop * damn! .... damn damn daaaamn! * pushed back more, about to be thrown away , had no hope remaining inside of him that he can stop the attack ... so ... * stoooooooooooooooooooooooooooooooop! * suddenlly uses his demon powers , his power goes throw the ground the goes up and stabs his parents .. , he feels pain at the same moment just like he was the one being stabbed ... he looks up a little almost can see anything but sees his father continuing even with the attack he just did .. * damn it .. * starts crying , looks at his father again .. stabes him more , but the dad keeps going and going and Nill had to keep the same .. but he couldn't stop his father till he blew up all the anger inside him on that father * ... * the attack on him starts to calm a little , but suddenly increased again! it was his Mom still alive and using her last breath * ... why .. * feels pain increasing much inside of him , shouts with all the energy left to him , tears of Blood falls on his cheeks , attacks again * whhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh­hhy­! * weakness suddenly stops all his moves , he couldn't stab his mother more than once .. but it was enough to take her to the sky ... alone ... with all what just happend ... he couldn't realize anything just stays in the same pose ... saying same words didn't know till when ... *
why ... I had to kill my parents?
they all aren't dead ... I'm the ... only dead here ...
I've ... just ... been ... killed ...

after that Nelian was known as the criminal demon that killed his parents ... he never said why ... just lived alone silently ...