How long until his heart breaks?

Has he crossed the line?

Stiles stood up straight no fear anywhere inside him. He began walking towards her heavily placing his feet up and down. She also began to move but began moving to the side and both people began circling each other as if eying for the kill.
"And what are you going to do skinny boy. You wouldn't hurt a fly" she began teasing him smirking.
"Well now I did kill your brother" Stiles said realising that he only felt bad when his friends faces were looking disappointingly down at him.
"Ouch my poor soul" she playfully slapped her hand against her chest, her smirk not faltering for a second.
Stiles felt a rage deep down inside of him churning. He has had this feeling before but never at this magnitude, it was like a thrashing sea inside of him which was finally ready to burst.
"You have no idea what you have released" Stiles eyes stared straight at Nyx no longer caring about anything beside her. His entire focus was now completely fixed.

He ran at her and punched her straight up in the jaw. She fell back and grabbed hold of a nearby pillar to steady herself. Stiles swung again but she dodged and Stiles wacked his hand on the pillar sending a wave of pain up his arm. He called out in pain before turning to look back at her. Nyx wiped her hand over her lips and looked down to see them stained with blood as Stiles had given her a split lip.
"Hm not bad for a baby" she nodded her head.
"I may be 147 pounds of pale skin and fragile bone but sarcasm is no longer my only defence" Stiles smiled seeming to enjoy the fight.
Both of them jumped back into battle swinging punches at each other.
They were both swinging aimless punches and were both pretty banged up before Nyx rolled away and pulled the dagger back out of her belt. She examined the blade carefully looking at her reflection before looking at the bloodied boy in front of him.
"Before you say anything I'm a fair fighter and you can't let your true nature out with just punches can you" she grabbed a second identical blade from her belt and threw it towards Stiles landing centimetres away from his feet.
He bent down and picked up the knife looking closely at the 5 inch blade. He turned his head and spat some blood specked spit from his mouth onto the floor then looked back up at her. His eyes a deep blue flickered in the dim light.
Nyx steadied herself and held the knife out in front of her waiting for Stiles to make the first move. He did.
Running towards her he held the knife dreading and strikes aiming for her lower chest. She blocked with her knife and scraped his off then counter attacked. Stiles moved back just barely avoiding a life threatening injury with the blade connecting with his skin and cutting down his upper arm leaving his free flowing blood flow gently down his arm to his palm.
Stiles looked up at his injury and resisted the urge to cry out in pain. However he couldn't feel it well as the adrenaline pumping through his veins made that impossible.
He focused back on Nyx and analysed the way she moved and acted.
He attacked again to her shoulder making sure her arms defended herself and in this moment swiped her legs. As she lost balance her arms went outwards and Stiles grabbed her hand and forced her to drop the knife out of her hands.
They both fell to the ground. She fell down her arms splayed out and reopened her eyes. Stiles was above her and had the knife to her throat.
"Kill her" a voice inside his head spoke.
Stiles almost gave into it. Looking at her shocked almost frightened face made something inside him smile, feel warm.
"You going to kill me" her initial shock wore off and she was now smiling.
"Tempting, very tempting but I'm not letting you win" Stiles said still trying to ignore the voice deep inside his head telling him to kill her.
He stood up and grabbed her knife so she couldn't attack him put it in his small belt and began to walk away. He could here her stand back up but he didn't dare turn around.
"So after all that your going to give up. You going to let me go and kill everyone you love" Nyx yelled at him.
Stiles hand gripped tighter onto his original knife.
"Maybe I'll make you watch. Watch as I torture you dad in front of you. Poor Sherif Stilinski he won't know what hit him" Nyx was about to say something else but stopped.
Her hand instinctively rose towards her neck. Warmth engulfed her hand and she felt something sharp pointing out of her neck. She grabbed it and pulled it out.
Her knife was in her hands stained with her blood. She dropped it and it clattered to the floor. With both hands she grabbed ahold of her neck trying to keep as much blood in as she could but it didn't work.
Within minutes almost all her blood had spilt to the floor. She followed crashing onto the floor head first and didn't move. A puddle of blood surrounded her but nothing also moved. Stiles heard her last spluttering breath.

Stiles stood there unemotional looking down at her body, his eyes like black flames.
"You talked too much".
He turned around and walked away smirking as he left the warehouse.