Have you studied the Three-Fold Law and the Wiccan Rede? Acoording to the Three-Fold Law, any thing you cast will come back to you three times the powers. Like the Law of Karma. It kinda prevents you to cast "evil" spells at people. Also, no spell or magic should mess with free will. So love spell and stuff are out of the question.
to me, i think that energy is neither createred nor destoried, so if you send out negative energy, it will still be there and it will come back. some goes to positive energy.
The basic one is to remember the universal law that everything you send out will come back to you. This happens as small as having a bad day, having bad feelings, dwelling on those feelings, and your day just gets worse, or smiling at strangers and watching them return it.. or as large as casting a harmful spell on another person and years later finding yourself greatly harmed or in another way made completely miserable. Once I cast a good luck spell on a friend. I had good luck for a few months.
You cannot change another's will. This does go back to Karma. Think about how you would feel if someone did that to you? Something in life would likely come to you that gives you those negative feelings. Not only is it not a good idea, and ethically wrong, but its nearly impossible.
Basically just think long and hard before you cast anything. Think of how you word things. Think of your emotions at the time you plan to project energy. Make sure everything is in check and you are satisfied with any possible repercussions.
And I would like to add that it is always a good idea and good ethics to ask before you cast a spell on another person. In most cases, the result you seek can be achieved by casting a spell on yourself instead. Always consider protection spells as an alternative to curses on others. The only times I have cast spells on others is when its a helpful thing for them and I have their permission.