Yes and no. They're not evil per se but their power is derived from their impregnation by a demon which the Shadowmen raise from the Earth. This explains comments such as Adam in 'Earshot' implying that the Slayers are demonic in nature, their affinity with various evil entities, Faith's temporary fall to the dark side and Buffy being tempted by the same in stories such as 'Bad Girls' and 'Buffy vs Dracula'. It also explains why Slayers seem to have a dark sexuality and a yen for older men with a darkside (like The Shadowmen) such as Angel(us), Dracula, Spike, The Mayor, The Master, The Immortal, Principal Wood etc