Let's get one thing out of the way right now: if you ask me, T-Rex is subscribing here more to egoism than ethical relativism. I'm not going to talk about egoism right now, but one of its biggest problems is that it is in a separate volume from ethical relativism in my Encyclopedia of Philosophy. They weight, like, five pounds each. It's kind of a hassle.
Moving on. Ethical relativism comes in many flavors, and they all taste bad. Let us start with T-Rex's flavor, that if A thinks it is right (or wrong) to do X, then it is right (or wrong) for that person to do X. If you believe this is true, then it is right for me to punch you in the face in order to convince you that it is false.*
The comic later gets into a different flavor of ethical relativism: there is considerable uniformity to moral standards across cultures, such as condemnation of homicide and cruelty. Granted, in some cultures, there are some circumstances where you are obligated to kill someone (e.g., vendettas and honor killings), but to kill a person without justification is always a no-no.
The issue becomes a little confused at the end of the comic, where T-Rex says that there are people out there who are okay with just about any atrocity you can name. If he wants to continue believing in his brand of ethical relativism, he will have to accept that there are people out there who think it is right to do things he thinks are wrong, and that he cannot judge them for this. I do not think this will last long, because when one gets down to it, one finds that there are people in this world who think it is right to do things that are unequivocally wrong.
For more on ethical relativism, I refer you to this excellent discussion.
* - Or is it? I leave this as an exercise for the reader.