Mourning is primarily about acknowledging feelings of loss for someone. If you knew the goy personally, then you feel loss. There’s nothing wrong with that, and you should be in touch with your feelings. If you didn’t, then for a Jew of course one should try to realize a sense of loss, for haskafic reasons – i.e. the Jew had a unique purpose etc. For a goy, I’m sure one could engage in a lengthy debate about whether his existence is worthy to us on a hashkafic level etc., but Jothar makes a good point.
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