The poem never says who its "public servant" is, and that's part of its game. The clues point in one direction, though: a man in a red suit who has seen "so many stories of death." The title and the red suit suggest a figure whose job is public and ongoing, and the poem's phrase "a most admirable workman" makes him sound like a tradesman who simply does his work well. Judging by the imagery, the figure may be Death himself, treated as a dependable employee.
The speaker's tone is oddly warm. Calling him "good company" turns something frightening into something almost companionable. Then the last stanza adds a wry twist: old age or the flag, two things people celebrate or fight for, come with their own "triumphs," yet they seem to be working for him too. The mood is calm, dry and a little darkly funny. Death isn't raged at here; he's acknowledged, even tipped his hat to, as the one job that always gets done.