A/N: I just saw SaiNW (thank you YouTube!) and they really never gave us a reason why Donnie left. They just said he did. So, here is my theory. 2003verse.
Miscalculation
By: LOSTrocker
Donnie's POV
I should've been more careful. I should have known better not to mess with it but I had to try. It was the only way we had to defeat the Shredder. I didn't know I'd be the one paying for it. I couldn't stay. I had to leave. I couldn't face my brothers. I was a freak as it was but I couldn't face them as a monster…
He was so close he could taste it. All the concoction needed was one more finale ingredient and then it would be done. Of course, it wouldn't be the real deal, but it was as close to the ooze as he could get. It was worth a try, it would be a risky one but if they pulled this off they wouldn't have to deal with the Shredder anymore. The ooze was part of their creation and Donatello was hoping it would be part of Shredder's destruction. They would use their enemy's own weapon against him. They would be the ones with the last laugh.
Donnie could feel his nerves. He was starting to shake. If something went wrong, there was no telling what would happen. Donnie stopped and took a couple of breath. Now, he regretted being genius of the group.
"Come on, last drop, and it will all be over." Donnie said to himself, hoping it would calm himself down. It did.
As careful as he could he went to pour the last bit in, but something happened. Looking back now, he wasn't sure what exactly caused him to be so careless but he was, he rushed. In the process, he lost his balance and he ended up pouring in a lot more then what the directions had called for.
Donnie cursed himself for being so stupid and the not so graceful one. He stepped back and held his breath. The ooze was coming to a boil. He prayed that it would be a dud. He could handle if he lost himself to this, but not if he hurt his family. He wouldn't know what to do. The bubbling ceased. Donnie let out his breath.
Nothing happened.
"Looks like its back to the drawing board." Donnie said, and just when he went to turn –
BOOM!
The lair went up in flames and smoke. He could hear the cries of his other brothers and his father. Then it was silence. Everything was still and black. It was the sound of a dropping rock that caused Donnie to come out of the darkness. True, it was still black but he knew he was conscious. Donatello tried to move but found he couldn't. He figured it was tons of rock on him, he was right.
Donnie let his hands roam around him, he felt the rubble. There was just enough space for him to where he could try to dig himself out, if there was a way out that was. He stopped a moment and found there was no other movement around him. A dreadful thought soared through his mind, his family was dead and it was all because of him. Yet, he refused to believe it. They'd over come a lot worse then this. They had to be around some how.
Donatello gone to cry out for them but found that he couldn't. The chemicals in the air seemed to have damaged his vocal cords. All he got out of a yell was a cough, and another one and another one, over and over. The coughing fit stopped. He could feel the tears coming, then a sound of hope stopped them.
"RAPH! MIKEY! DONNIE!" That was Leo, or at least he figured due to the roll call. It sounded like Leo easily enough but he could hear that there was some damage in his brother's voice as well.
Donnie tried again, but a coughing fit came out instead.
"RAPHIE! LEO!" Mikey was the first to answer the roll call. "DONNIE!"
"Over here Mikey, follow my voice!" Leo instructed.
Donnie could hear movement. He knew it was Mikey due to the clumsiness in the noise, and the constant "Owws".
Donnie was waiting for the third. Come on, Raph… He pleaded with whomever was listening. Someone seemed to be. Raph made his presence known easily enough.
"Dammit! That's gonna hurt for weeks."
"RAPHIE!" Mikey cried out. His voice that was once high pitched and squeaky like was now sounding more rough, almost as Raph's was. Donnie worried if his brothers continued to cry out like they did, they might have any voice at all by the time this was all over.
They were all accounted for, at least the brothers. "Splinter?" Donnie heard Mikey ask. "And Donnie? Any word from Donnie?"
A light blinded Donnie's vision, it was coming from the north. He made out a familiar shadow of a rat. "Splinter!" he heard Leo cry.
"Come my sons!" Splinter ordered. "We do not know if there will be another blast. We must make haste!"
"But what about Donnie?" Mikey cried out, nearly in tears.
"You three go!" Splinter ordered.
"Splinter-" Leo was arguing, which wasn't the best idea.
"Don't disobey Leonardo." Splinter warned his eldest. "Take your brothers to April's, tell her what happened. Go!"
Donnie knew Leo didn't want to go but he heard the scuffling. They were gone, that meant Splinter was here looking for him. God, he tried, he tried so hard to make some sort of noise anything, but he couldn't. His voice wasn't cooperating with him and he could find nothing to through to get his father's attention. All seemed lost.
Then a glare caused Donnie to wince his eyes. He found a piece of broken mirror near him. It wasn't much, but it was something. He took it in his hand. He went to through it but when he went to through it, he caught his reflection in the small piece of the mirror. His reflection wasn't one he recognized. If he could've screamed, he would have.
Thanks to the exploitation, the chemicals not only damaged his vocal chords, but also his physical appearance. It was if his face had simply melted away, as well as part of his upper body. He hung his head and cried. The tears caused him to wince as they landed on his fresh wounds. These scars would never heal. It was bad enough he was all ready a mutant, now he was a monster, he couldn't bear it. Donnie couldn't let his family see him like this. He had to leave. So, he waited until he heard one more finale cry from his father for him to come to him. He wanted to remember his father's voice. He remained still until he heard the footsteps disappear. Splinter had chose to leave. Donnie didn't blame him. He was right to order Leo and the others out. There was a possibility this place could go up in flames again.
Donnie waited a few more minutes just to be on the safe side. He didn't want to get caught. Once he thought he'd given himself enough time, he tried to move again. It took all his strength to flee from his once beloved home, but was able to do so quickly. It was mostly Splinter's teachings that helped him with this: "Seek hard and fade away, without a trace…"
Splinter would return again, but he wouldn't find his son here. His son would be long gone. Nothing would be the same again…
Fin…
A/N: I might continue this, might now. Still debating.