Donatello was the third sign.
Donnie was quiet, and smart. Very smart. He helped her with her homework, showed her the things he created, chatted with her for minutes on end. He was a bona fide genius, gentle and sweet, so naturally he caught a lot of shit from his family, particularly Raphael. It was a special pleasure for the red masked ninja to reduce his brother to tears for not liking to fight and tinkering with his computers.
April felt bad for Donatello. She tried to stand up for him, as much as she could. In later years, his lab became her sanctuary whenever his brothers acted less than normal.
It took her longer to start seeing his problems, but when she did, it was all at once, and it was horrifying.
She had stepped into the lair to find it completely silent, and the front area empty of life. "Guys? H-hello..."
"Hi, April."
"Donnie?"
He was leaning against the counter where he hadn't been seconds before, smiling crookedly at her. When her heart rate slowed enough so she could talk, she exhaled and cocked an eyebrow at him, gesturing to the rest of the lair. "Where is everybody?"
"They're not around today. They're not feeling very good."
"Oh. They're sick?"
"Mm. Pretty horribly, too, I might add." There was something in his tone and in his hazel eyes that worried her, something dark and amused. "R-really? How come you didn't get it?"
"Easy, I gave it to them." he commented calmly. April froze in shock, finally noticing the two empty vials on the counter next to his elbow. "C-come again?"
"I poisoned them, April. It won't kill them, but they'll wish it would. They'll all be shitting and vomiting for the next two days," he laughed.
"W-why, Donnie, oh my God..."
"Raph and Mikey thought it would be funny to fill my pillowcases with bags of blood, and Leo didn't stop them. So this is funny to me." Donatello crossed his ankles and grinned at her like a naughty little boy as someone moaned in agony, and there was the sound of painful vomiting followed by the toilet flushing. There was no remorse in his voice, or sympathy.
"You don't feel bad?"
"No. Did you still need help with your Trigonometry?"
After that, Donnie continued to poison his family randomly, even if they hadn't done anything to deserve it, claiming he just wanted to see what would happen.
She was at home and in bed one night when the thought stuck her that Donatello fit one of the terms she'd learned that week in her psychology class- Sociopath.