But if you follow the expected model of 4-8 encounters per day, that's still a lot of adventuring. Based on the pace of the game, that could still be one or two sessions. Sometimes the party can rest, sometimes they can't.
Yes, it's a stretch. An exaggeration. But they're not completely different situations from the perspective of someone in the world.
In both cases, you can bring the person back given enough time. You can save your friend.
The difference is one of scale. If it's okay to murderer your friend to stop them from being an inconvenience for a couple months, the why not do the same over a couple weeks? Or why not a couple days?
It might be hard and they might have to make the sacrifice, but shouldn't they at least try?
And that sounds like a good story. Of struggling but failing to save a companion. Or choosing one companion over another. Or choosing to fail in a mission to save a friend. That's a story and memorable element of a game and far, far more interesting than just murder.
Is it evil? Kinda. Evil can be just as simple as putting your needs above someone else's. It's not that they're unwilling to sacrifice themselves for their catatonic friend, but that they're not even willing to try to save them. Anyone can be good and noble when it's easy. That doesn't count. It's when you make the right decisions when things are hard that sets your alignment and shows what kind of person you are.
We weren't given that information. We have to work with what we were told, and what the OP presented was the party immediately moving to murderhobo the situation.
The OP did not say they immediately went for murder. It can be read that way, but I would assume they at least asked about alternatives, rather than deciding, without consulting DM or the Player of vegetable, that they should kill vegetable. Also, I said it could be considered evil, depending on their in-character reason for the kill. As I said, if their only excuse is really "I don't wanna" then yeah, that is evil. But there are situations ingame where I feel the choice is acceptable.