Snape is under a lot of stress and he lashes out. He hates Harry and wants to kill him, but he is under orders. Voldie wants to kill Harry personally. Snape must also impose discipline on the other Death Eaters. In the heat of battle, other DEs might forget their orders and kill Harry. Plus there is the debt: James saved Snape from werewolf!Rhemus; Snape is under some sort of obligation to save Harry.

All this pressure and insults too. I think Snape would have lashed out whatever the insult was.

Courage is the Gryff virtue. Slyth's virtues are purity, ambition and cunning. "Coward!" = "You are not a Gryff" is a very lame insult to aim at a Slyth; "Half-blood!", "Minion", "not as cunning as Draco" would hurt a Slyth more. Maybe James yelled "Coward!" while he tortured Snape and the memory stuck.

Snape and Petunia had painful childhoods. They brooded on the pain and became bitter and worse. Harry and Luna had pain and are good. Dumbledore's sermon about moral choices in CoS finale.