I wrote a prompt the other day asking what would happen if Snape forces Harry to feed a potion to Hedwig during lesson, the same way he forced Neville to feed a potion to Trevor. Most people had the, quite ordinary and expected reaction that either Harry would refuse or Snape would be punished/murdered/eaten by Lovecraftian monsters. However, some people defended Snape in the situation with Neville and Trevor and it struck me as … odd.

So, I went back and read the scene and thought about it some more and came to the conclusion that not only was Snape definitely willing to poison/kill Trevor, the scene has another layer that I've never seen before.

First, Snape was definitely willing to let Neville kill Trevor and was upset that he didn't. Literally in the text is it written that Snape was "sour" faced that Trevor survived. This is a negative reaction. He isn't frustrated or annoyed or disappointed that Hermione helped Neville, he is upset that the potion worked. When he told the class they were about to test Neville's potion his eyes were "glittering". Here is every instance of Snape's eyes glittering:

COS: "But why not join the feast afterward?" said Snape, his black eyes glittering in the candlelight. "Why go up to that corridor?"

Suspicious and mistrustful.

PoA: Professor Snape was sitting in a low armchair, and he looked around as the class filed in. His eyes were glittering and there was a nasty sneer playing around his mouth.

Cruel and annoyed.

GoF: "Antidotes!" said Snape, looking around at them all, his cold black eyes glittering unpleasantly.

Unpleasant, demanding and angry.

"Do you know what this is, Potter?" Snape said, his eyes glittering dangerously again.

Dangerous and threatening.

"What is this rubbish?" said Snape, his black eyes glittering. "What are you talking about?" (RE: Crouch Sr. appearing before Harry and Krum.)

Dismissive, disbelieving and deliberately obtuse.

Snape is not a nice guy and his eyes glittering is not a good sign.

Some people were arguing that Snape was actually trying to motivate Neville to try harder (which isn't just bad pedagogy, but really fucking stupid) or that he knew that the potion was going to work by looking at it (this is an invention by the reader and his reaction doesn't support the idea), but these people were adding their own ideas into the text. It's all well and good to remember that Snape is secretly a good guy at the end of the series, but that doesn't mean that everything he does is secretly good. I'll get to his actions at the end of PoA in a moment, but he still refuses to recognise Hermione's injury to her teeth in GoF ("I see no difference." Was this part of his secret masterplan?) Plus a dozen more examples. Snape may secretly oppose Voldemort (oops, spoiler warning), but he's still a cruel, petty, vindictive bully.

But the second point, and when I realised it, it gave me the impetus to write this, is that the Trevor scene is a parallel for the climax of the book. Snape seeks to punish someone innocent, (Neville(Trevor)/Sirius), but Hermione tries to talk him down. Snape ignores her and goes through with his decision anyway (feed the potion to Trevor/summon the Dementors to perform the kiss). Unbeknownst to Snape, Hermione has a trick up her sleeve (secretly giving Neville instructions/travelling through time) and has thwarted his plan. After this Snape exacts a petty punishment (five points from Gryffindor/getting Lupin outed and forcing him to resign). There's even a parallel that Hermione must give her help secretly.

Rowling loves to write like this, giving the reader subtle clues as to what is going to happen that can only be seen on a second reading (all the fake divination prophecies coming true. The centaurs predicting Harry's death at Voldemort's hand in the forest. Petunia knowing about Dementors from "that horrid boy", etc.). I'd give it a 95% probability that this was exactly what Rowling was implying in the scene with Neville. This is Snape's plot for PoA in a nutshell.

That being the case, you cannot argue that Snape was acting rationally with Trevor, without arguing that he was acting reasonably with Sirius, when he was deliberately ignoring and refusing to listen to evidence that could have proved Sirius innocent and instead tried to get Sirius' soul sucked out. Snape was willing to let Sirius "die", therefore, he was willing to let Trevor die. It's like poetry…

Snape is an interesting character and a great addition to the books, but, especially in PoA, he is an evil bastard. He acts bravely and in opposition to Voldemort, yes, but he is not kind and he is not good. He deserves everything bad that happens to him for the way he treated Harry, Neville, Hermione, Sirius and Lupin and that's just in the third book.