The title, "Internecine," means destructive conflict within a group, and the poem seems to apply that idea to a single person. Each stanza begins with "who," so we never get a clear subject, only a figure described in fragments. He "seems safe," yet he is "broken in pieces." The outward look and the inner reality don't match.
The strangest line is the one calling him "aught but an argument against himself." He isn't fighting an enemy; he is his own opponent, a set of contradictions that keep disproving each other. The last stanza adds a twist. He "will not exchange his own head," which could suggest stubbornness, or perhaps a odd loyalty to his own mind, even though that mind is the source of the trouble.
The mood is cool, spare and a little unsettling. The speaker doesn't seem angry or pitying, just observant. The missing main verbs leave the portrait unresolved, as if the self-division can't be finished or fixed.