To be fair, even if the potion would not kill Trevor, it was particularly cruel to let Neville believe he would have to kill his own pet. I don't hate Snape, I think he's a great character, but the way he treated Neville is the one thing I can't forgive him for. Maybe it's a personal thing since I had teachers who would bully students in a similar way to how Neville was treated and I guess I just can't get past that.
I feel I should probably begin by stating that I am not advocating his approach, or saying that his method is one that should be celebrated or emulated, but conversely, I am not convinced it’s wholly unreasonable in the scene with Trevor.
The key factor is that Snape is attempting to incentivise Neville, and the evidence for this in how Snape approaches OWLs:
“Moronic though some of this class undoubtedly are, I expect you to scrape an ‘Acceptable’ in your O.W.L…”
He also says:
“I advise all of you to concentrate your efforts upon maintaining the high-pass level I have come to expect from my O.W.L. students.”
We don’t know why Snape takes such a pride in his high-pass level. (That’s a whole other meta.) But it is enough that we know that he does, and we know that Umbridge regards his class as being advanced when she assesses his teaching – so we know he’s not lying.
That helps to explain Snape’s stance; he wants Neville to succeed, because he doesn’t want to lose his high pass level.
Onto Neville. It’s important to remember that we, the reader, are privy to lots of information about Neville because we travel with Harry, one of his Gryffindor dorm mates. We fully understand that Neville is a nice lad who is terrified of Snape, and his fear causes him to make more mistakes. However, it is fair to argue that perhaps Snape wasn’t aware of this.
So what does Snape see?
We don’t see any other child struggle in quite the same way that Neville does. The other Gryffindors might dislike Snape, but they don’t seem scared of him. I’m not suggesting that Snape is beyond reproach, but I think it possibly hasn’t occurred to Snape that Neville is too scared to ask for assistance. I think Snape genuinely can’t understand why Neville can’t follow simple instructions:
“Orange. Tell me, boy, does anything penetrate that thick skull of yours? Didn’t you hear me say, quite clearly, that only one cat spleen was needed? Didn’t I state plainly that a dash of leech juice would suffice? What do I have to do to make you understand, Longbottom?”
So Snape sees a Gryffindor kid who is either not listening, not concentrating, or is being deliberately obtuse. He doesn’t ask for help, or clarification – and then he fails to brew his potion correctly.
Snape’s threat is not kind, but I think it’s the behaviour of a teacher nearing the end of his tether. We see from Neville exploding his cauldron in his first lesson that Potions is dangerous. It’s perhaps apparent to Snape that Neville doesn’t see that he’s putting himself, his classmates and even Snape in danger by not brewing correctly (or he will do in the future as the brews become more complex), so he ups the ante and puts something – Trevor - Neville genuinely does care about in danger.
As Hermione manages to salvage the potion, we know it wasn’t a lost cause, so it’s unfair to suggest that Snape wanted him to fail; it was possible to succeed. The real discrepancy is between what Snape thinks is happening (Neville deliberately not listening and not engaging in class) versus what is actually happening (Neville being terrified of Snape and not being able to engage in class).
I think there is a part of Snape that is genuinely - even if he does it in a horrible way - trying to get Neville to improve. So even if Snape knew that Trevor would be fine, it would rather undermine his whole approach if he said, “I’ll poison your familiar, but don’t fret, he’ll be ok,” - even if Snape was certain of that himself.
It doesn’t make Snape’s treatment of him correct, but it does perhaps explain a little where Snape’s frustration stems from within the Potions classroom.