Jack describes what happened on Halloween from his point of view. Why did he say those mean things about August?

From the text:

I'd been talking to Julian about August. Oh man. Now I understood! I was so mean. I don't even know why. I'm not even sure what I said, but it was bad. It was only a minute or two. It's just that I knew Julian and everybody thought I was so weird for hanging out with August all the time, and I felt stupid. And I don't know why I said that stuff. I just was going along. I was stupid. I am stupid. Oh God. He was supposed to come as Boba Fett! I would never have said that stuff in front of Boba Fett. But that was him, that Bleeding Scream sitting at the desk looking over at us. The long white mask with the fake squirting blood. The mouth open wide. Like the ghoul was crying. That was him.

I felt like I was going to puke.