Well, I won’t write you an essay, but I’ll give you an overview.
I appreciate Snape’s bravery and loyalty to Lily. I don’t think anyone can look at his dedication to her and not admire it. He had a hard time growing up, and it made him bitter, and sad, and had him clinging to the thought of Lily even more. She was a source of light in his life, something good that he hadn’t yet ruined. Until, of course, he did and I think that he spent the rest of his life trying to make up for that.
However, I am not the biggest fan of Severus Snape. Although I appreciate his dedication, I think that that he often acted like an immature child. Sure, James Potter tortured him at Hogwarts. But he was a sixteen year old boy that was trying to impress the girl that he liked, by any means necessary. Snape kept up that mentality even when he should have already grown out of it. He treated Harry as if he was James himself, and frankly acted like a brat around him. He was never able to get over that trauma of his childhood, and it’s sad that he couldn’t get past that to really mature.