Damon was rather angry, He knew that Elena's friends shouldn't stick around him when he was mad. The vampire wasn't always nice either. He didn't want to kill any of ELena's friends.
But one just happened to be in the wrong place at the wrong time. When Damon was angry, his hunger was more likley to be out of control. The vampire growled and glared at Elena's friend.
He knew they would never be able out run him. He lunged and killed them before they had they had the chance to get away.

"No,' Damon whispered when he realized what he had done, He backed away. He wasn't sure what to do. He knew he had to tell Elena. He considered hiding it. But he knew that she would be even more angrier with her than she would be if he just told her. The vampire was sure either way she was going to be angry.
He wondered if he had just ruined his chance at being with her.