I know there a couple of second level save-or-die spells, like Aboleth's Lung and Blood of the Martyr, but are there any first level ones? I'm interested in any spell which, on a failed save, guarantees the death of a valid target barring action on the part of that target, not just spells which explicitly kill the target on a failed save.
I do not believe there are any first level spells that actually kill the target outright. There are, however, two first level spells which, on a failed save against a valid target, immediately take an enemy combatant out of the fight as if they had died, and opens them to coup-de-grace.
It's very limited in its application (4 HD maximum, living creatures only), but meets the criteria for a Save or Lose: Those who can be affected must make a saving throw or guaranteed death by coup-de-grace, unless woken up by an ally.
Like Sleep, Color Spray's save-or-lose portion is limited by HD, but the effect is similar, and falls off gracefully to an AoE stun at later levels. Unlike Sleep, an unconscious character cannot be woken by an ally.
To my knowledge, no. Save or die spells are extremely niche and there are none at a level earlier than two or as effective as Glitterdust. First level spells aren't meant to have a level of power capable of killing anyone, so unless it was a homebrew spell or some imbalanced 3PP material I doubt you'd see such a spell at first level.
And while Aboleth's Lung certainly can kill someone if they fail the save due to suffocation, its limited to the Gillmen race without DM permission. Blood of the Martyr has a conditional statement that allows the person bleeding to reduce their wounds by one half. They can also just drink a healing potion of any type to end the spell effect. It's quite a weak choice for a spell.