Damian doesn't understand why people are so stupid. What's the idea of Halloween? A bunch of stupid kids going to ask for candy that might be drugged. Or maybe he's bitter about never having participated in a Halloween, but the reasons don't matter.
If it's for candy, it's very easy for him to ask his dad or Alfred for candy. It's much easier than knocking on a door and waiting for the person to give you candy.
"Halloween is stupid, so no, I'm not going to ask for candy with you." Damian rolled his eyes. "And what are you dressed up as? You look pathetic." He said staring at your costume.
No, it doesn't look pathetic, but he has no reason to be nice to you. You've never done anything against him, at least not directly.
Damian is a little jealous of you, actually.
While you were growing up in Gotham, having a normal life most of the time, he suffered in the training of the League of Shadows. He knows that you have daily training sessions with Dick or Bruce, and he knows that they don't get mad at you, but with Damian the situation is different.
Probably because he always tries to be a smartass and gets angry with the advice he gets. He's surprised that Bruce let you go to Halloween.
A few days ago Bruce asked Damian to take you trick-or-treating, since Damian refused, he thought you couldn't go, he was even happy with the thought.
But here you are, dressed up and bothering him.
"Get out of the way, if you want to trick-or-treat, ask Alfred." Damian said giving you a push, not with all his strength, of course you wouldn't be able to handle it, but strong enough to make you stagger.
He sat on the couch and put his feet on the coffee table while he picked up the video game controller.
"Go to that witch convention already, you're bothering me." He said as he grabbed a bucket of popcorn and put it on his lap.