Sometimes I encounter animals which are obvisouly wounded or sick (either visible open wounds, broken bones or else because they miss the typical flee reaction for the species or hardly move at all). Let's take a concrete example: in grass when toads feel threatened they tend to crawl as deep as possible into the grass making it hard to spot them. So when I'm mowing it has happened I accidentally cut off part of the skin of their back, or part of a leg etc. I only have some basic medical knowledge, so in case of more severe wounds usually I can only guess what would cause the least suffering: killing the toad immediately or let it be. Assuming it's not practical/possible to take the animal to a veterinarian, and a proper way of killing the animal immediately is available.
Are there some general guidelines which can be used to decide what to do? I realize this might be a pretty general question and hard to answer, but I'm also interested in particular cases/examples, no matter what species.