I think people tend to be to sensitive towards malfoy. yes he was crying and the effects of ptsd were beginning to hit him. but in Harrys defense a war is going on during this time and Draco is on the other side cursing and hexing people throughout the whole book and film. Harry did not know the spells effects and made a mistake but at the same time I think he felt justified in punishing a deatheater. even if it was malfoy. anyway the best part of this scen is Alan Rickman as Snape and his reaction to seeing Harry use his spell on someone. no words. straight experession. great acting.