Guns are directly linked to the actions of those bad actors. A private tunnel of communication is not so directly linked to the actions of child abusers (nor the images they share on these platforms).
Also, distributing child porn is itself a crime, separate from the abuse. So you would be directly using E2E in a crime that way. Are you implying that some laws hold less moral or legal value? If so, what are the differentiating factors in that decision?
Using the function of a gun on oneself is not a lawful use case, just as selling a human is not a lawful use case for WhatsApp. A more apt comparison using your example of not writing anything to encrypt would be the possession of a firearm compared to the possession of an E2E system. Both have lawful as well as unlawful uses.
On the part about writing the message being the first part of the crime and comparing that to committing suicide with a gun, the gun is the means by which the crime was committed, just as the means for the crime of selling the person would have been the medium through which it was transacted - WhatsApp in this case.
Perhaps you where trying to explain something else?