His Fatal Weakness
Severus Snape sneered snarkily to himself as he sliced lacewing flies that needed to be added to the Shrinking Solution he was making in approximately three and a half minutes.
Sometimes he swore to himself that Longbottom got worse at Potions each time he tried to make one, often resulting in melting the cauldron and causing some kind of boil, rash, or excessive fever that made a trip to the Hospital Wing utterly necessary.
The boy made him want to vomit.
Severus had always been good at Potions—the impeccable timing that was needed and the necessity of precisely chopped, diced, or sliced ingredients had fascinated him from the moment he could read. From that time onward, he had looked forward to being able to go to Hogwarts to learn the precise art of Potion-making.
Little did he realize at that age that, to be a completely independent Potion-maker, one was required to be able to grow one's own ingredients if they were plants.
Severus soon realized this, of course, and began to look forward to Herbology as well.
But his first Herbology lesson didn't go as well as he had hoped. Oh, yes, it went well at first. Severus tried to make the cuts to the Snapping Daffodils as precisely as possible so as not to kill the abusive plants, and Professor Sprout graded him well for his work.
At first.
Needless to say, Professor Sprout wasn't as pleased with him when it was discovered, a week later, that all of the Snapping Daffodils he had handled had died unfortunate deaths.
His grade in Herbology dropped from an 'O' to a 'D' in his first week at Hogwarts, and it only got worse from there.
Certainly, he could do the theory perfectly, but it didn't help that every plant he touched withered, and the professor soon forbid him to touch any of the plants.
Poor Severus could barely scrape an Acceptable.
To this very day, Severus had to rely on other people to grow the plants he needed for his delicate potions. Once they were harvested, they caused Severus no inconvenience at all.
When they were supposed to be dead.
So, yes, Neville Longbottom was the source of hatred from Severus Snape.
That that utter imbecile who could not control his magic or make a decent potion if his life depended on it could do what he, Severus Snape, could not, was infuriating.
And Severus did not handle that well at all.