You'll tap and transform Homicidal Brute even if it couldn't attack.

If Civilized Scholar attacks, and later in the turn (but before the beginning of your end step), it transforms, Homicidal Brute's last ability won't trigger. This is because the creature attacked that turn, even if had its other face up at the time.

You don't have priority between untapping Civilized Scholar and transforming it. You can't activate the draw-and-discard ability again, for example.