So, what’s interesting here. First:
People left unchecked — this is a short participle (Past Participle).
The structure noun + Past Participle (irregular verb, or if regular then with the ED ending) describes the state of the noun.
Literally: “people who were left without checking/control”.
An equivalent in English is the participial phrase “people left without control”. People [who are] left [and are] unchecked
a broken window → someone broke the window (result of an action).
a written letter → someone wrote the letter.
people left unchecked → people were left in a state where they are not checked.
Second: Will do evil — here will is not used for the future tense, but to indicate a confident prediction or natural inclination. The phrase means that people have a stable, predictable inclination to do evil in the absence of restraining factors. It should be treated as the phrase: “People will constantly do evil.”
Third: is composed of — a fixed passive construction meaning “consists of”, “is formed from”. A more formal synonym for is made of.
How are such constructions formed?
Subject + [verb “to be” (am/is/are)] + [Past Participle] + [preposition]
He is known for his generosity.
The government (subject) + is (verb “to be”) + composed (Past Participle of “to compose”) + of (preposition).
The team is composed of experts.
The table is made of wood.
Paper is made from wood.
