Am I wrong to use bronzer as a contouring tool?
In a word, yes. The difference between bronzing and contouring is quite vast and as a result, so are the tools and products used for the job.
Bronzing is used when you want to look more sunkissed, as though you've caught some rays on a summer walk if you will! It is best positioned on what we call the high points of the face - to explain, they are the bony, prominent areas of the face - the bridge of the nose, the cheekbones, the chin and the two sides of the forehead. When you are applying the product, ideally use a flat topped brush to hit the planes of the face correctly and thus distribute the colour best.
The difference in a bronzing powder or cream is that it has a more bronze/red base to mimic the shades we naturally go when we tan. If you use this as a contour under the cheekbones, along the sides of the nose and under the chin as well, the main thing that will happen will be that you have this bronze tone all over your face! Thus cancelling out your foundation tone (which no doubt, you've gone to great care to get right, right?)
Contouring products have a different purpose and a different effect. Firstly, they need to have more of a beige-y brown base - not dark brown or in any way orange - that's what's being used when we all react so strongly to bad contour! Dark tones, make you look like a tiger, soft beige tones work; they offer subtle shading and shaping and therefore a good effect. Brush-wise, contour brushes are best flat with a slanted or rounded edge and should be as small as a small blush brush. Any bigger and you run into the problem outlined above - product all over your face, rather than in the relevant specific area!
So, to answer the question, yes, you are wrong to contour with bronzer I'm afraid. Some things in makeup application can have multi-use but bronzing and contouring are not such an example. When done right (but separately) they can give wonderful enhancement to the face and can co-exist quite happily in a makeup routine.