It depends. Generally speaking, it cannot be bad to spy on someone, because even if you find out something, that you wish you never knew, it is still the truth - whether you know it or not. Spying on a cell phone is usually done by parents, who want to monitor their children's activity, and even more commonly - by people, who think their spouse is cheating. In both of those cases, mobile tracker can be very helpful in the long run - in the short run, you may find out something, that makes you feel really bad, but ultimately it is better to know the truth. Also, consider this: 95% of the things we worry about never come to pass, so you may also be worrying completely unnecessarily.