Just so I understand, you want me to explain why it is just to wiretap child pornographers and murderers. Is that right?
Whether it's wiretapping or execution, when referencing those crimes you're making a personal evaluation of the actions without referencing any criteria for for how you judged them. To do so is to play to people's emotions about the crimes you referenced rather than the actual item for discussion, which is when wiretapping is justified, and specifically for what crime.
I view it as equally manipulative to state "we can all agree that murder is bad and wiretapping should be justified in some cases" as to say "we can all agree that changing traffic lanes without signalling does not justify wiretapping" when the actual question has nothing to do with either crime.
This is just a case of Godwin's law writ small, so it's harder to spot.
(An alternative argument is that for any extended powers of the state, we should have statistics to back them up. If wiretapping is ineffective for a crime or it's benefit is outweighed by it's downsides, maybe that should be taken into consideration. In the end, I'm basically I'm for questioning generally unquestioned positions.)
In short, if you just say "who cares about suspected murders, probably most of them are murderers", you are leaving a lot of discretion over your life and the life of those in society around you into the hands of those whose power it is to enforce the law. And while they might be the best people for the job, ultimately they are just people too.