I've heard the phrase I wouldn't piss on X if they were on fire from time to time. It's pretty self evident what it means.
Now think about it for a second. If I detested somebody so much that this phrase would be applicable to them, then I definitely would piss on them if they were on fire.
Here's my reasoning. If the fire is too big, then simply urinating on it can't possibly put it all out. On the other hand, if the fire is small enough that urinating on it would be sufficient to extinguish it, then it would be easy to aim the stream away from the flame and direct the it to non-burning parts of the body.
Either way, it seems to me that the best course of action when confronted by a detestable individual, laying aflame on the ground, is to urinate on them. If only to add the insult to injury.