Because according to the file, if I bite an imPort, then they don't turn into normal walkers. Every imPort will get a different strain of the virus that I have, and that will give them change them into a potentially powerful walker. Walkers are hard enough to take down even if you know how to handle them - and we all know just how hard it is to fight a normal imPort.

[ But just in case if Tetsuo doesn't understand, Carl puts it this way - ]

Look, If I bite Pan, what if he turns into a walker that won't die of a headshot? What if he can still use his powers even if he forgets who he is and only wants to eat living flesh? What if the only way we can beat a turned Pan is a way we would've never would've guessed, like dousing him with water or have to cut off his fingers and burn each one individually?

That's why I don't want to risk biting imPorts. And if I do . . . it's better to put a bullet to their brain and quick before they die and turn.