The first time Harry Potter set out to end someone's life, he failed. He didn't understand what had happened, not until much later, when things could be sorted out and explained to him by others, when his emotions could be pushed aside. In the moment, though, there was too much noise, too much shouting, shock and horror and guilt over the inflexible determination to stay alive, even if he didn't deserve to, even if he should have died. That was what Lavender had screamed at him, shrieking it while crying and trying to attack him with her fingernails. He hadn't even stepped back, hadn't reacted in the slightest. After facing down Severus Snape and any number of Death Eaters, he just did not have the capacity within him to back down in the face of a girl's grief. Besides, she had the right to be angry.
Hermione had more right, because Lavender was an ex-girlfriend and Hermione was his true love, but Hermione, though white and pale, was holding Lavender back, preventing her from attacking Harry and then stolidly accepting the other girl's fists striking her.
That moved Harry into action, because it wasn't fair that Hermione be blamed. Ron was dead, and it was Harry's fault.