None of the actual dangers of magic have actually been described thus far.
Spells do not backfire, they are just ill conceived. Examine the possibility of what could happen to the greatest extent, prepare yourself for every possible outcome, and understand that magic follows the same rules of cause and effect as anything else.... If you wander out there thinking everything will be just as you wish it just because you wish it then yes, spells might "backfire," simply because you are too short-sided to see more of the possibilities that might result from your actions.
The law of three, where what you put out there comes back to you three fold, is a wiccan thing. Many people do not believe in it. I do not believe in it. Sure, the old saying "You reap what you sew" always applies. This does not mean that the universe will come back and haunt you for what you've done. It just means that if you sew discord and vengeance... that's what you get. If you create a peaceful world around yourself with love and so forth... well, that's also what you get.
You will not kill yourself with magic. I don't know what's wrong with the people on this site, but they seem intensely gullible and melodramatic about such things.
Most damage that you might experience in actual magical practice is psychological in nature, and even then it is typically self inflicted ("I know magic so I'm all powerful and now very delusional"). Understanding yourself, possessing confidence within yourself, and understanding what you practice will go a long, long way in helping to prevent any possible harm.
Just study, practice, and don't believe all of the crap people say.