Try outputting to another monitor or even TV and see if that works OK, via RGB or HDMI outputs if you have them onthe Chromebook. That'll help narrow down whether it's the screen or some other issue.
If it's a 1st time run and it seems it is, it can take a while. Maybe give it overnight. Of course if there's something evil lurking on the machine, that's another story... But give Malwarebytes/BitDefender more time first?