If an important distinction between people and animals is that animals only take pain up to 2, then a human who perceives pain only (mostly?) at the 2 level might be more like an animal.

In terms of experiencing pain, yes (although I do think there are more level 3 animals than Craig does). If I had to choose between torturing a level 2 human or a level 3 human I'd pick a level 2, providing the torture did no lasting damage to the body.

However, a far, far more morally significant distinction between a human and an animal is that humans can foresee the future and have preferences about how it turns out. I think an important part of morality is respecting these preferences, regardless of whether they involve pleasure or pain. So it is still wrong to kill or otherwise inconvenience a level 2 human, because they have many preferences about what they want to accomplish in life, and thwarting such preferences is just as bad, if not worse, then inflicting pain upon them.

It would even be fine to inflict small amounts of pain on a level 3 human if doing so will prevent a major life-goal of a level 2 human from being thwarted.

EDIT: Of course, I'm not saying that no other animals possess the ability to have preferences about the future. I'm sure a few do. But there are a great many that don't and I think that is an important distinction.