Is that what happened? Did the FBI hack some guy's computer for no reason then watch him jerk off to midget porn thru his webcam? Or did they set up malware on a child pornography website that tracked people that visited the site so they could find them and prosecute them? From the article, it seems this is what they were doing.
The feds aren't sitting around hacking into random people's home computers fishing for something they can arrest somebody for. That would be totally illegal and a violation of your 4th ammendment rights. They're tracking people that visit a child porn site. I have zero problem with this. This would be no different than staking out a child pornography bookstore, running the license plates of the people that go there and then use that information to find them for prosecution.
What you are basically advocating for is that if the reasons for disregarding our governing laws meet a personal threshold then so be it because they were catching bad guys. This is why we have a constitution. It's not optional otherwise why have it if it only applies to certain people in certain circumstances?