This. The OP has stuck the words “as individuals” into his question, even though those words are not in the quoted text from Donne and, indeed, the whole point that Donne is making is that we are not just individuals; we are all “involved in mankind”, and the collective dimension of the human experience is just as central as the individual dimension.
A couple of people have posted to say that they could, hypothetically, rejoice at certain deaths. Perhaps so, but that doesn’t mean that they wouldn’t be diminished by them, in Donne’s sense. Donne doesn’t say that every death makes, or should make, you unhappy; just that it diminishes you.