how about female cops. young, hot, athletic. because, hey, how about a challege (and i have a thing for uniforms). when word gets around that there is a serial killer who targets policewomen next target will be harder to get, because they will be more cautious (don't make them stupid teen slasher characters, make it a challenge, and remember that cops usually come in pairs). also, the whole police force will be after you. and media will go crazy. i think it's a good premise for a thriller...
this killer does not hunt them with a rifle from distance, he gets up close and personal. he could torture them with their own weapons (taser to tits, for example), leaves their naked bodies for the cops to find and collect their badges. there you go.