It certainly is awful to come across a severely injured animal on the road. I have way too strong a sense of self-preservation to tackle anything large, including snakes. But occasionally I have come across things that were small enough to put out of their misery. A blue-tongue for example that had been run over and left to die with it's innards hanging out of its mouth. That one was so awful I shed a tear driving away, I'm such a soft touch. And rabbits are everywhere here, and always getting hit. Although they are a massive pest and rabbit roadkill is no loss to the environment, I still can't handle seeing them suffer.
Thanks for the advice snake-catcher, I'm sure it goes against the grain to tell us that, but I do think it's important to know.