I'll be very interested to see what kind of answers you get to this. I've never been much of a bag person, I go for practical (for years I carried a canvas one that could get dirty on the floor of the car after hiking or mountain biking... that kind of practical - eek!) and I've never really "got" spending a lot of money on one, after all, very functional ones are available at reasonable prices. I also find I get tired of them and want to swap them out frequently, so like to feel I have that option, guilt free. And you may have noticed rarely are my bags part of my OOTD pics... it sits by the door, not in my room, and gets grabbed as I leave the house. I do have some nice statement ones, but I don't change them every day depending on my clothes. Esp in the winter, it's more about going with the coat. I guess in the summer months they are neutral enough this isn't an issue, but I would make a change if there was a clash.
However... I'm coming around. I absolutely see how they are an important part of an outfit, and fairly easily add another element. Esp "killer" ones :). And I'm finally figuring out what kind of bag works for me, so getting a more expensive one is becoming less scary.
So, to your question. To my mind, a "killer" bag is a real statement. Usually in a colour that stands out against the outfit, not brown or black, but fushia, citron, that kind of thing. And it would probably be structured, wouldn't look "worn in" even though I do like that look - it's just not "killer". It would be art, not just a purse.