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I don't understand the mental state of people who vandalize message boards.
Literal
Message-board [object-を] vandalize people [of-の] mental-state [subject-が] not-understand.
掲示板 'bulletin board, message board' originally referred to physical bulletin boards but now primarily means internet message boards in modern usage. 荒らす 'to vandalize, lay waste to, troll' has a specific internet use for disruptive posting/flaming. The whole noun phrase [掲示板を荒らす人の精神状態] is the subject, marked by が, with 分かる taking its object in the nominative. Expression of mild outrage at online bad behavior.