As seen here, he says he is going to kill them and seconds after you can hear the screams. The reason the act itself isn't shown is because it's still a comedy that includes some younger audiences.
Why he did it can be two different reasons or a combination of it. He might have done it to sate his appetite for blood, he is a vampire after all. And the hippies are the best target he could get, seeing they were roamers without any ties to the community. If he had killed a local police would investigate causing him troubles...but a band of roaming hippies, who would even know that they were gone?
The second reason might be that he did confide some of his past/curse in to them, something he is supposed to keep a secret for his plans to restore the family pride to work.