Weeping Angels do not need to kill people, however they prefer to send the human in question back in time, feeding off their potential life energy. An angel may indeed kill people though for use of their body or to survive.The Weeping Angels "kill people nicely;" "let them live to death" by sending them back in time to feed on the energy of the days they could have lived. Basically, they feed on potential energy. They kill people mostly to satisfy their hunger. However, sometimes they have certain goals that they'd like to reach, such as getting in the TARDIS, and would kill someone because they got in their way.