Harm isn't intended to work as an instant kill.
While it's ambiguously worded, the D&D 3.5 FAQ p.83 clarifies the design intent of the spell. Harm cannot reduce you below 1 hit point even if you fail your save.
Q: The harm spell deals 10 points of damage per caster level (to a maximum of 150 points at 15th level) and cannot take a target's hit points to less than 1. If the target creature makes a successful saving throw, the damage is reduced by half, but the spell still cannot reduce the target's hit points to less than 1. [...]
A: Technically, that's right. [...]
This makes sense if you consider the D&D 3.0 version of the spell, which reduced you to 1d4 hit points no matter the total. D&D 3.5's method instituted a damage cap but retained the spell's inability to work as an instant kill.
Consider also that it says "but it cannot reduce", rather than "and it cannot reduce", suggesting that this clause is not an additional benefit of passing a save.
However, as the FAQ notes, you can happen to die from the little-used Massive Damage rule, which requires a DC 15 Fortitude save any time you take 50 damage from a single source. The FAQ recommends ignoring massive damage from harm as a house rule, since harm is not intended to be an instant kill. Every D&D 3.5 group I've played in has ignored the Massive Damage rule in general.