let it go across the street

and buy wards (i lol'd at that)

get a snake and feed it

sell it to someone on ebay

Duel time! Put Soraka your pet goat versus The rat!

Give it a crossbow and three crossbow bolts. Then wait for it to evolve...

If you want it to have a peaceful non violent death you could turn your car on and either hold the rat near the exhaust or leave him near it in a little cage. He will pass out from carbon dioxide/monoxide and die peacefully in a slumber.

I remember a story years ago I read on the internet that a cat owner recently had a litter of kittens and one of the kittens suffered a terrible accident and he put it down that way. I was appalled by his actions for a long time but I realize now he did the right thing given the extent of injury he described.

put it in a pokeball and train it, as u are its master xD