Cursing correctly has no negative effects on you unless you already feel it isn't right and basically curse yourself. Another possible negative effect is cursing someone connected to you in some way (work, school, or family) because that connection can cause it to effect you as well. The only negative effects to any carefully crafted ill intending magick is obviously the fact that you are embracing negative emotion, thought and energy which can cause imbalance and draw negativity to you in your life and spiritually. But this only occurs when the reason behind it is selfish. Cursing a rapist for example because they have raped you (or someone you love), many other women, and will rape more in the future...is an act of justice and forceful prevention of further victims.
Let's look at it another way. Say you see someone wielding a gun and shooting people. This person may have a good reason in his own mind (each side of a war believes they are right, the winner writes the history).
If you call the police, since it is obviously harmful and violent, the police come and shoot him dead on the spot. No one thinks this is odd, life goes on.
If you pull out your own gun and shoot him, you may even receive an award for being a hero.
If you pull out your own gun and try to be Rambo (injuring a few innocents) because you're taking the opportunity to use your gun and this excites you, you're considered an idiot and you're just as bad as the 1st guy with the gun.
If you call on local criminals, they'll shoot him, maybe killing a few innocent bystanders and leave. Now it's a tragedy and you're just as bad as the criminals. The local criminals welcome you into their group (whether you like it or not). By the way, they like to turn on each other too to see which one is stronger, so good luck with that.
This is the way harmful magick works.
If you call on higher beings that specifically judge people, the actions that occur afterwards is their choice and not yours. It doesn't weigh on you because you brought it to their attention and let authority decide.
If you do it yourself, it depends on your reasonings and how carefully/intelligently you do it. If its seen as just, it will be supported by the positive spiritual beings. It's done with a good heart and not as an excuse to hurt. It's usually obvious when it's done out of a desire for power over a person or reason to harm out of anger/hate. "They cheated on me and now I want them dead!" And there comes the negative aura and law of attraction.
If you call on malicious forces, they'll either desire payment or expect you to be their best buddy. I find it hilareous that people will cast a curse then complain about a negative entity in their home and banish it. They probably just spit on the one who helped them. Then suddenly their curse "backfires". Hmmmm, coincidence? I think not. Foolish is more like it.
As Crowley said "Do what thy will, that is the whole of the law". And by "will" it is the universal will (not individual will). And universal will is what is best for the whole of life.
I don't advocate cursing, but there is no stopping someone who wants to do it. It's like telling a teenage boy not to have sex. You know he will if the opportunity arrises, so its best to assume they will and guide them. In the case of the teenage boy, you give him a condom and talk about possible dangers like STDs.
The curse I posted remove the negative emotion from the individual placing it externally into a "battery" of sorts. This provides them with a clear mind where they can think logically "why am I doing this" and "what is the best approach to solve this". Some people do need a kick in the backend. It's just that most people go overboard and focus so much on themselves and not what is the best and most beneficial action for the whole.
Curses can be found anywhere on Google. It's not a "secret" and if they want to find one, they will. It isn't a curse they're after when they post here about them, they want guidance. And when they specifically say they don't care about law of three, karma or such as this one "save me the garbage". Just saying "it's bad" in so many words isn't going to do any good whatsoever.
