simple

3.

He knew exactly what it was about her. Not that is is advised, but he didn't know how else to start describing her, so he made a internal list:

She was beautiful. Incredibly so. It was not something anyone reasonable would find difficult to admit. Her green eyes mostly glared, but they were symmetrical in a way that shocked you and made you reconsider the path you were about to take. She stared through your eyes, into your brain. They were perfectly shaped, somehow. Her movements were guided by a natural gracefulness nobody else seemed to have when she was around. He had never seen her smile or even grin, but her neutral face gave him the impression that when she did smile, it would be world-shifting. She was always flawlessly dressed, but would always wear simple pieces that made her beauty stand out, rather than her clothes. He liked her dark eyebrows that looked thick and combed out, and he had always admired the way her hair looked shiny, even in the dullest of rooms. She never spoke in class when she didn't need to, which made her voice carry some weight and her words have meaning, whatever she said. Everything she said flowed out of round, plump lips. She was pale but had freckles, like organised splatters of paint on a canvas.
He could go on for a long time.

Secondly, she never seemed to have cared much about him. This was good, given his past.
She had been against The Dark Lord.

Voldemort. Voldemort. Perhaps this was the most important reason and should have been the first. For a Slytherin pure-blood to be so opposed was groundbreaking. Rare, one could say. No. Draco actually thought it to be impossible before he saw her. It was as if he had understood some great concept when he laid his eyes on her, even if it had been a subconscious understanding. Something clicked in his mind, even if at the time he did not realise it. The sight of her kneeling in the Hospital Wing would never leave his mind.

It was a long time before he figured out what Hera had done; what she had accomplished. His thought process at the time of The Battle was:

"She is lowering herself down to the level of these filthy mud-bloods, half-bloods and enemies and wasting her time by saving their life. She's a fucking traitor. Their life doesn't matter anyways. Neither does mine. What's the point? Why doesn't she save herself some time, get up and come here? Fight for a noble cause?"

Meanwhile, most of the students of House of Slytherin lay silent underneath the war that was waged above. Never questioning the power of their own blood purity; they sat 'preserving' their noble selves. The students deemed themselves more important than their parents, even, who at least were fighting and then dying to serve blood purity.

Say what you like about the purity ideal, but at least some fought for it.

It was not a commendable feat in the end, but at least they chose a side and stuck with it. Their children, however, used their lineage as merely an excuse not to fight, and consequently, an excuse to stay impartial. Impartial was the most dangerous position of them all.

Impartial meant traitors. Actual traitors.

These types of individuals always would be on whatever side served them best. Always safe, but never fighting. Not even siding with their own family.

Draco told himself that, at the very least, he had chosen a side.

The wrong one, but one nonetheless.

He had to live with himself now. 

Meanwhile, Draco watched his mother protect Harry Potter, risking her own life.

Meanwhile, his father retreated behind a wall of blinding shame and anger and killed himself shortly after The Battle ended, like a coward, not being able to accept a loss.