Facts and figures underscore the degree to which guns function as an immediate threat to the lives of countless innocent citizens.
Shouldn't threat be plural since guns is plural?
Threats would be correct since you are also talking about a number of citizen's lives.
In this sentence, the use of the word "threat" is singular because it is referring to the overall nature or characteristic of guns as a danger to innocent citizens. Although the subject "guns" is plural, the writer is discussing the collective effect of all guns as a single concept – that they function as a threat. The word "threat" is being used in a more general sense to describe the potential harm or danger posed by multiple guns, rather than referring to individual threats posed by each gun.
However, if the writer wanted to emphasize the idea that each individual gun represents a separate immediate threat, they could use the plural form by saying "function as immediate threats to the lives of countless innocent citizens." This would highlight the notion that each gun carries its own potential danger.