Maybe Carragher thought the same, that he would never spit at someone and that people that do are despicable.
On the other hand, many people who think spitting is unforgivable would think it's "normal" or ok to slap (or even punch) someone who does something to them that they feel is unaccaptable or disrespectful.
Maybe Carragher is one of those people. But seeing as he was in his car, and his arms aren't 10 feet, in his moment of anger he chose to spit at the man in return for his taunts, for him he felt the man deserved it. When in need (and especially when in anger), you use the cards you have at the time. In his case he had multiple cards, but could only use a few, like: 1. car, 2. distance, 3. anger and 4. Pleghm. At least he didn't use the card "car" and crash into the man
If the same situation arose but with the two men (and daughter) in a mall or walking on the street, and Carragher still chose to spit at the man, I would maybe judge him more harshly.
I'm going to stop now before I sound more like a snot and scouse loving bastard, hope somebody understood the point I'm trying to make.
And no, spitting at someone is not (one of) the worst things you can do to a person.