Hi, sorry it had to come to this, I remember this from the chat.
A small herp like that is instantaneously and painlessly killed by destroying all brain activity simultaneously.
The way to do so is massive head trauma. It sounds horrible but the idea is, the newt will never know what happened. Lights out.
I suggest enlisting your dad (or someone you trust with a strong stomach) to deliver a massive blow to the head with a hammer or similar object. This will obliterate the skull and brain instantly, so instant lights out.
It's tough, but sometimes these things happen. I wish you better luck with any future herps you may decide to keep.