Raph was the second sign.
The second oldest was in a constant state of violence. If he wasn't hitting something, he wasn't happy. Sometimes April would visit and not see him for the whole day, only hear him beating the living hell out of his exercise bag, punch after punch after punch until his knuckles were swollen and bleeding. Only when that happened would he be content.
If the punching bag wasn't handy- he went through about three a week- then anything else would do, including the refrigerator, the tv, anything breakable in Donatello's lab, the walls, the chairs, and his brother's skulls.
Especially April.
Raph would slap her, pinch her, kick her, pull her hair and overall abuse her, delighting in making her squeal in pain and passing it off as 'just messing around'. He LOVED to torture her to no end, like she was his own personal little stress toy.
Raphael took sibling squabbles and turned it into something frightening and painful. He gave Mikey so many concussions and broken wrists, April thought it was a miracle that the youngest could write and tell her what state he lived in.
But Raphael despised his oldest brother most of all. Anything that gave Leo joy, he went out of his way to destroy. He told him he hated him at least once a day, and the feeling was clearly returned.
Eventually, the abuse grew more harsh, and his fits of rage more frequent until finding Michelangelo unconscious in the kitchen was a daily thing.
Raph refused to watch anything without death, blood or murder involved, and talked about those things constantly, usually reserving the goriest of his talk for the dinner table.
As they matured, his language worsened, especially towards April. He started making comments about her body and the way she looked, and more than once she caught him staring at her with a vicious, lusty light in his golden eyes.
His "play fighting" got even more physical. He'd twist and pin her arms behind her back and force her to the ground, and laugh at the sight of her squirming and sobbing beneath his weight.
"OW, OWCH! Lemme GO, you psycho!"
"Ya wanna get up, Red? Say mah name."
"YOU'RE HURTING ME! I can't breathe!"
"Say it! Say mah name!"
Once, when April spent the night, she went to the kitchen for water and found Raphael still up, watching a brutal rape scene on the tv and chuckling while he jerked himself off.
The girl on the screen had red hair. His moans increased with her screams, and April heard him mumble her name once or twice.
Raph came hard when the man cut the girl's throat, and April wasn't thirsty anymore.