Helping a total stranger who approached me on the internet "hack" into someone's myspace or other social network account? How would I know I wasn't assisting a stalker, rapist, pedophile, serial murderer, or other type of predator? Yeah, ok.
However, parents have not only the right, but even a responsibility to monitor their childrens' computer activities to protect them from potential internet predators. Companies have the legal right to monitor their employees' computer activities--protection against wasting company resources, fraud prevention/detection, and to make sure their employees are not engaging in illegal activities for which the company could be held legally liable.
As far as cheaters go? Well, unless they've actually made the committment to each other to live together, I would regard it as an invasion of privacy. For those who are living together or married? Well, at least in my state, you have the right to do whatever you want with your OWN property, your own computer, in your own house--including monitoring it. Most people who suspect their spouses are cheating on them are usually right. And if they're capable of cheating and possibly planning on leaving you, good possibility they're also capable of at least attempting to "hide/steal" marital/community assets before they go. Like, being cheated on isn't bad enough, do you have to let them rob you too?
So if someone asks me to help them hack into a myspace account? Well, those who have a legitimate reason for desiring to do this should also have physical access to their target's computer. There's keylogger hardware devices which they can legally place on their own computer(s) for the purpose of monitoring it, and by doing so, can have accesss to any and all email, Myspace, and other social network activity that transpires on it. Keylogger hardware is 100% undetectable by ALL antivirus programs, fits nicely inside laptops via mini-PCI card, and can be attached on desktops with a tiny device placed on the keyboard cord.
From an ethical standpoint, this is the advice I provide.