Here are thousands of instances of "our life is short", and I seriously doubt many of them are specific to the lives of, say, a couple living together. It's perfectly normal to refer to human lives collectively in the singular.
A plural subject requires a plural object (lives), accordingly a singular subject requires a singular object (life). They can be used to mean one person or several people, however. So, if your intention is to refer to an individual who lost his or her life whilst saving the lives of more than one others, then the second is correct and the first incorrect. They is singular in this context, so ...
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