Misa is the most frightening character in Death Note. Far more than Light. Light kills people with a reason, for a purpose, with actual thought and intent and ideals. Light won't snuff you out unless you do something wrong, or try stopping him from attaining his goals. Teru, crazed fanatic that he may be, at least kills people out of a sense of justice. Even the Yotsuba Kira killed with a mind towards profit, using calculated actions to eliminate competing businesses. Misa just kills people for fun.
No thought goes into her slayings. They aren't done for any purpose, and she has no personal ideals. Instead, she kills people because of the little girl crush she has on a serial murderer.
Misa is young, sweet, innocent, and the purest imaginable evil.