I don't care if he's got a Dark Mark or not; he's spent all of HBP trying to do something for Voldemort; this aligns him with Voldemort if earth logic is to be trusted. I'm also quite aware that other outcomes are possible, such as he stays neutral, runs away to the Azores, or hides out in a Manchester sewer until it all blows over (and has frequent tea parties with the local rats).
Here, I'm asking only about reasons for Draco Malfoy to switch sides and be actively involved on the side of the Order of the Phoenix. Yes, you can pick more than one if you feel some reasons are equally "most realistic/interesting/favourite". That's why I used the ticky boxes. :D
ETA: The question isn't meant to be "consider the canon, and while you're doing that, I'll be over here stealing all your base." It's more along the lines of "if we only have canon to go on, and any of the below happens, how realistic/interesting/etc would it be, in your opinion?"
What do you think is the most realistic (given canon only) motive for Draco to switch sides in the Second War?
What do you think is the most interesting (given canon only) motive for Draco to switch sides in the Second War?
What is your favourite motive for Draco to switch sides in the Second War?
I ran out of ticky boxes. Very sad.