You unleash a wave of necromantic energy that snuffs out the life force of those in its path. This wave pulses out from you in a line 5 feet wide and 30 feet long. The wave visibly rips the souls from the bodies of those it passes through, which manifest as screaming, transparent versions of the affected creatures. The wave kills every living creature of 17 or fewer HD in the line, starting with the creature closest to you, to a maximum of 1d4 HD of creatures per caster level. No creature of 18 or more HD can be affected. If a creature succeeds at its saving throw or has too many HD, it doesn’t count against the HD the spell can kill The wave continues to affect creatures as it rolls away from you until you either run out of HD to affect or reach the limit of the spell’s area. If the spell does not kill any creatures, the unreleased necromantic energy violently explodes in the final square of the 60-foot line, dealing 10d6 points of damage + 1 point per caster level to any creature in that square with no saving throw. If several creatures occupy the same square, roll randomly to determine which is affected.
Emphasis added
I'd treat it as the explosion, rather than the instant kill, at that range.
Basically, the thing gets unstable after 30 feet, and it is just a bomb waiting to explode for the rest of its distance. So it fully destabilizes when it hits something or reaches 60 feet- whichever comes first.
Alright. I was more so curious if that wasn't an oops. It's kind of unique as a spell that way.
It was likely an oops. I just suggested a functioning version through speculation.