His death was foreshadowed through the series, and we know that the Shadowspawn armies will most likely attack in ToM according to the blurb. Min even had a Viewing of Nynaeve "kneeling over someone's corpse in a posture of grief" in TGS. It would also nicely allude to the title "Towers of Midnight" if the last "King of the Seven Towers of Malkier" dies alongside his fellow Malkieri in a stand against the Shadow (though Nynaeve might become pregnant yet, and give Lan/Malkier an heir, as also another of Min's Viewings suggests). Finally, I think it fits in a bittersweet way that at the time when Moiraine returns from the dead, her former Warder dies, so that they won't ever see again.