Severus Snape was never expected to live.
His mother considered ending his life before he even breathed his first. She almost stayed a part of her proud, pureblood family. She almost didn't marry the man who would make the next twenty years of her life a living hell. But she did.
He was a sickly baby. Eileen almost died giving birth to him. That's why there were never any more children. The doctors said he couldn't live, he shouldn't live. His lungs were too little. Eileen ignored the muggle butchers, but secretly wished for them to be right. They weren't.
His father nearly beat him to death when he was nine. Severus hid under his bed for weeks after that, hugging his broken arm, afraid to be seen during one of his father's drunken rages. At night, when he was forced to listen to his mother's screams, or fell asleep with an aching, empty stomach, Severus wished his father would just go ahead and end it. But, of course he didn't.
His Master, Voldemort, thought nothing of him when he first joined. He was just another foot solider. Disposable. He saw more action in those first months than many of the others. He saw many of his fellows brought down by his former teachers and classmates. He knew, he just knew, that one day one of the curses would meet its mark, and he would not survive. He did.
He thought of ending it, when Lily died. He brewed endless poisons, created hundreds of spells, just to end his life. He thought that one day, his loyalty to Dumbledore would subside and he would be brave enough to take his own life. He never was.
His Master, Dumbledore thought too much of him when he first joined. He sent him into places no sane man could go into and come out alive. He trusted him with too many secrets. He counted on him for too many plans. Severus knew, he just knew, that one day someone would discover where his true allegiance lay, and he would not survive.
Severus Snape was never expected to live. And he didn't.