The boy went mad. The verb in this sentence is-

Linking verbs are verbs that serve as a connection between a subject and further information about that subject.
They do not show any action; rather, they “link” the subject with the rest of the sentence.
The verb ‘to be’ is the most common linking verb but there are many others e.g. become, seem, appear, taste, smell etc.
In the given sentence, ‘went’ doesn’t mean the action of going; it serves as a link between the subject “The boy” and further information about that subject “mad”.