The phrase suggests you're wearing clothes to make a big impression and get noticed. At the fashion show, he was dressed to kill.
Pro Tip 2/3
Not Literal
It's not about physical harm; it's about making a strong visual impact with your clothes. She entered the room dressed to kill, turning heads everywhere.
Pro Tip 3/3
Special Occasions
Usually used when someone dresses up more than the everyday, for an event or outing. For his graduation, he was dressed to kill.