Figure of speech question

[cattiechaos]

I recently came across a figure of speech that makes no sense to me: "I wouldn't spit on him if he were on fire." Apparently this means that you hate someone so much, you wouldn't spit on them if they were on fire. I don't see how this makes sense; NOT spitting on that person would be the polite thing to do, no? After all, spitting on them will not put out the fire. In my mind "I wouldn't spit on him if he were on fire" = "He is such a wonderful person, even if he was on fire I couldn't bear to spit on him in the hopes of putting out that fire because it would be a cruel thing to do." Can someone explain this figure of speech to me?

Okay I don't know if that will make sense to anyone else.

Are there figures of speech (in any language) that didn't/don't make sense to you? If English ones are strange to me, I can't imagine what those in different languages will be like.