Ethics in cooking? The title seemed strange to me. I had never thought of ethical cooking. Once I thought about it, there are several obvious things that should be mentioned.

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The golden rule applies to cooking:
"Cook for others the way you'd like others to cook for you."
Respect your ingredients
Use fresh food.
Take an interest in:
Where your ingredients come from.
How they were grown/raised.
How they were harvested/slaughtered.
How they were treated/butchered.
Respect your guests
Prepare food to the highest standards of cleanliness possible.
Communicate ingredients and cooking procedures clearly to those who eat your food.
Be mindful of possible allergies and dietary restrictions. Eating some foods can be life-threatening to some people.
Never include mood/mind/body-altering substances without the informed consent of those who will consume the food. This includes:
Herbal supplements like ginkgo, St. John's wort, etc.
Alcohol.
Pot Pizza, magic brownies, etc.
Take into account the dietary needs of those who'll eat it:
Religious restrictions (like no pork for Jews and Muslims, no beef for Hindus, etc.)
Self-imposed restrictions (such as vegetarian considerations)
Doctor-prescribed dietary restrictions
Allergies and intolerances
Respect the environment
Try to buy local food. Freshness is half the secret of good cooking and local food is normally fresher. Be aware of "food miles". However, bear in mind that things may be more complicated than they seem. Fuel consumption is not always related to distance. It takes less fuel to transport an apple by ship from the other side of the globe than it does to take one by lorry (truck) from a few hundred kilometers (a couple hundred miles) away. Food sold in supermarkets as "local" is likely to have been transported from the local producer to a distribution centre far away and back to the supermarket again. Cut out the middleman: try to buy directly from the local producers. Buy only from retailers you trust. Don't get over-enthusiastic with buying local. Many poor countries' economies depend on the export of fruit. By refusing to buy tropical fruit, you could be contributing to poverty and environmental destruction in those countries.
Try to buy seasonal food. Again, seasonal food is likely to be fresher than non-seasonal. Apart from the environment, you will be respecting your pocket and your tastebuds. Use common sense. Do you really need to eat strawberries in September when you are missing out on the best grapes in the year?
Avoid over-packaged food - there's still nothing better than a brown paper bag for holding mushrooms, for example.
Use a shopping bag - the planet is filling up with once-used plastic bags.