There are several reasons why being a vegetarian results in good karma. First, it is saving a life. That counts as real good karma. Remember, you get rewarded or punished according to your deeds. Saving a life generates a pretty big reward. It could very well be the reason why someone "miraculously" survives a devastating accident. Karma doesn't count a rabbit as less than a human (so slow down on that back country road at night). A life saved is a life saved.
Now, we all know that in the animal kingdom, animals survive by eating other animals. Or rather, animals with sharp teeth and claws survive by eating animals with dull teeth. That's the way they do it. Lions live in prides, but they don't have a neighborhood farmer's market where they can get some tomatoes, a head of lettuce, and a bag of apples. Humans, on the other hand, have the option of avoiding meat. A diet of broccoli, spinach, tofu, and other vitamin-rich plants can provide more than enough of the same essential vitamins and minerals (and none of the animal fat) humans need to live well. Plus, you're saving a life. Henrietta Hen doesn't have to get plucked and have her head chopped off for that bucket of KFC chicken. Paulette, that cute little pig that has the intelligence of a small child, doesn't get turned into bacon or ham. Even if you're not an animal rights person, it's obvious that every animal wants to live. Eating meat violates that most basic of survival instincts for the sake of a full stomach. Sounds kind of selfish, doesn't it?