Yikes. That's a tough one.

My neighbor's cat dropped a baby rabbit with a broken back on my front porch once. I just couldn't cope. I had to get my other neighbor to come take care of it. I am NOT good at things like that.

Car exhaust is a terribly cruel way to die. It is hot, and burns the lungs, causing slow, painful suffocation. Not humane.

I would have probably put the rat in a cage of some sort, and tried to find a rehabber or a shelter to take it the next day. If that didn't work, I'd probably find some guy to kill it for me. I'm a weenie about that sort of thing, and so is my bf, so he would be no help.