Donatello was so wrapped up in trying to break away from what he was...no...is, he forgot to step back and really think. Think about his decision. Think about the consequences, not just the benefits. The benefits that honestly were probably chances with luck and fate.
But that was the thing about Donatello. When he was so wrapped up in something, he more often than not ended up hurt in some way. He may be intelligent and a man of science, but science doesn't always mean it gets along with fate or destiny. It was as though the mutant laughed in the face of fate. Like he could control it. Like he had forever to live and laugh at life because he was so intelligent he could easily outsmart the simplest of atoms.
And love did that to you, too. Make you forget how to be yourself. How to properly think. How to wake up and smell the probability and science in the morning, not just chance.
Perhaps it was the need to get out of the lair and be more than just a thing hiding in the sewers that drove Donatello to make an anti-mutagen to make him normal, perhaps it was the want to be normal so he could be with the beautiful April O'Neil, but either way there was SOMETHING that pushed him to find a cure for the thing that made him this...monstrosity.
Donatello worked into the late hours of the night and the early hours of the morning working on the potion that could either end him or make him the one thing he had always wanted to be: human. Finally he could walk the streets without having to hide in the shadows of the buildings and take shelter given by the night. Finally he could go to school and one day win that Nobel Prize he had always wanted. Finally he could have a normal life and maybe even live the rest of it with the red head he loved.
And with time, he had created the potion that could make or break him. Looking at the substance, Donatello felt the sudden feeling of nerves for the first time since he had been wrapped up in the project. He was the only one who could make a cure, well, besides April. But if it had the opposite effect than what he hoped...
Donatello shook his head. He couldn't go back. Not when he was this close to success. Not when he was this close to being human. The mutant moved the test tube in a 'cheers' like movement before tilting his head back and chugging the substance.
He waited for something. Anything. Donatello started to feel the change and his heart sped in excitement. After a moment of the feeling, he tasted blood in his mouth and could feel his stomach hitch.
Something was wrong. Horribly, horribly wrong. Donatello dropped to his knees, coughing up blood into his hands that dripped to the floor. But he was so sure he had gotten it. So sure he had finally made the cure. Where did he go wrong?
The mutant's thoughts spun in his head, trying to think of what he could have gotten goofed up. Maybe he didn't put enough of a certain substance in? Too much? But how? He was always right about these kinds of things. Always.
"Donnie? You in here?"
Donatello coughed but couldn't cry out to the sweet voice of April. He felt himself slump against the wall, feeling his vision begin to blur. He couldn't believe he had been so stupid. He had been so wrapped up in wanting a human life, he had been too cocky about his work and was blinded by his arrogance. Arrogance that finally got the best of him.
"Donnie!?"
He could hear footsteps race toward him and felt two small, soft hands on his skin. They eagerly brushed against his brow and onto his cheek. He opened an eye to see how scared she looked. How worried she was. How worried she was for him.
"Donnie? Donnie! I'm here, it's okay, you'll be okay I promise. Oh, god, there's so much blood...what happened? Donnie, stay with me, okay? You have to stay with me. Please, Donnie. For me."
He wanted to. So badly. But he could feel himself slipping away as his insides were being torn apart deep within him. Tears pierced Donatello's eyes and he wanted to say so much to her right then but couldn't form the words.
April looked as though she wanted to run for help but didn't want to leave him alone. 'Oh, god, please don't leave me alone.' Donatello leaned toward her and she held him to her as he became limp against the floor. She held his head and grazed her fingers over his bald skin in a soothing way.
"It's okay, it's okay, everything will be okay...I promise..." She repeated the phrase over and over, but deep down Donnie knew it wasn't. He could hear her scream for help. For someone. Anyone.
Donatello felt himself plunge into darkness. He could still feel April. Hear her. He wanted to touch her. See her. He was scared. He didn't want to die. Not like that. He could make out faint words that broke his heart:
"Stay with me, Donnie. I love you."
Donatello felt a sob rip through his chest. If he hadn't done this, he could be with her just the way he was. He didn't have to be human to be with her. Didn't have to be human to be the great man he was. Is. He tried to say "I love you, too." Did she hear him? Did he say it? He wouldn't know, for he plunged the rest of the way into an eternity of darkness.