Someone concealing their identity is doing it for nefarious reasons.
Spiderman conceals his identity
Therefore, Spiderman has nefarious reasons.
As a syllogism, this is valid:
A(someone concealing their identity) is(doing it because of) B(nefarious reasons)
C(Spiderman) is A(someone concealing their identity)
Therefore, C(Spiderman) is(doing it because of) B(nefarious reasons).
However, this syllogism is not true, as anyone familiar with the character of Spiderman can tell you. While the second premise is true (Spiderman does, in fact, wear a mask), the second one is not: Spiderman wears a mask to protect his enemies hurting the people close to him, something that is generally agreed to be far from nefarious.
The logic that J. Jonah Jameson, as well as many other authorities in the Spiderman mythos, use to justify their persecution of the web-slinger is an example of a logical fallacy, more specifically a fallacy of sweeping generalization. The first premise takes something that is true for some parts of a category, and makes into something that is true for all of a category.
In this case it takes the category of people who conceal their identity, and says that it’s true in all cases that they have nefarious reasons for doing so. While this is true for some them–bank robbers, for instance–it is certainly not true for all. While this is a valid syllogism, it is neither true or sound.
Also, J. Jonah Jameson can be little bit crazy. Look at those eyes. It’s terrifying.