The best and fastest way to kill your Camellias in containers is to use a potting soil that is formulated for bedding plants or vegetables. The reason for this is that Camellia root systems like a soil that gives the roots a chance to breathe. They will not tolerate soils that are compact with no air space. This is common with bedding plant soils.
Our Camellia Soil
We grow all of our camellias in a pine-bark based soil mix. The bark is finely ground, less than 1/4" pieces. Sometimes sold in bags as mulch or soil conditioner. We also use an organic compost and builders sand.
The ratio we use is roughly 90% bark, 5% compost and 5% sand. You can play around with it to find the numbers that work well for you and for your climate. Wet climates where the plants will get a lot of water may benefit from the lower compost numbers, but very dry climates may benefit from more compost. It all depends on where you are and your conditions.
Alternatives:
You can check with a local nursery that grows their own plants and see if they can recommend a soil mix for your camellias.
You should also be able to use any bagged mix for shrubs and trees. Avoid bedding plant and vegetable soils. Also make sure there are rocks or other materials in the bottom that will allow the water to drain off. Empty saucers after watering.