That's a good question. As a DM, I would rule that yes, Breath of Life would work in this case. Death effects are a specific type of attack, usually due to some foul necromantic mojo at play. They make it harder to raise the unfortunate fallen character and as a result are particularly scary for adventurers to encounter. I think it would cheapen the fear these attacks can inspire in your players if every big damage dealer effectively gets free death effects on his attacks as well.
That being said, if you like being a hardcase with your players then go for it. Just keep in mind that Massive Damage is already generally harder on the PC's than it is on their foes.
When optional rules are added, one should expect funky interactions :)
I would say yes, it works because it's still just death from regular damage, and I would count the dead creature as at negative hp equal to its constitution score plus the amount by which it failed it's save.(or maybe half damage taken in the killing blow, I don't know)
Massive damage is not a death effect.
Dying due to coup de grace has the same issue.
I don't see massive damage as a death effect.
That said: what really matters is the condition of the body. Massive damage represents a blow powerful enough to kill you WITHOUT chewing through your hitpoints. Nobody is going to survive if you whack their head clean off, no matter how many HD they have. Unlike Resurrection or even Raise Dead, Breath of Life does nothing to change the condition of the body aside from granting it more hit points. Yeah, you could probably Breath of Life someone who died due to massive damage, but they might be brought back sans-head, or cut in half from shoulder to waist, or missing their arm at the shoulder, or torn neatly in half by a dragon bite.
I don't see massive damage as a death effect.
That said: what really matters is the condition of the body. Massive damage represents a blow powerful enough to kill you WITHOUT chewing through your hitpoints. Nobody is going to survive if you whack their head clean off, no matter how many HD they have. Unlike Resurrection or even Raise Dead, Breath of Life does nothing to change the condition of the body aside from granting it more hit points. Yeah, you could probably Breath of Life someone who died due to massive damage, but they might be brought back sans-head, or cut in half from shoulder to waist, or missing their arm at the shoulder, or torn neatly in half by a dragon bite.
That is an interesting point of view Nails. Does this mean that cure spells don't fix broken bones, punctured lungs, or open wounds? Because I would say they do. Any of the physical damage which would prevent the body from maintaining like would be resolved. Or else cure magic would be mostly useless. I mean if someone breaks their arm you can restore their HP but their arm is still broken? Sorry that doesn't make sense to me.
The headless effect is actually vorpal which I believe is more like a death effect...the weapon enchant actually uses circle of death to make...so yeah I could see it not working on vorpal but massive damage and vorpal aren't the same.
RAW as written breath of life would work for massive damage...obviously it is the DM's call.
That all goes back to what hitpoints actually represent. To my mind it's plot armor, not actual physical toughness. If you're level 1 or level 10 getting stabbed in the belly is the same injury unless some pretty hefty changes have been made to your physiology.
An enemy swing that takes off 10 HP doesn't necessarily mean that it physically took a chunk out of your character, it just came THAT much closer breaking through your defenses. A critical hit or the hit that takes you from positive to negative hitpoints is a hit that connected with real force, maybe breaking a bone or severing a limb.
Take Die Hard for example (not the feat). John McClaine is being shot at by drug dealing nazi terrorist hijackers. Two of them shoot and on screen you see both shots miss. Mechanically though, one rolled below his AC and "missed" in game terms. The other hit his AC but the hit wasn't enough to kill him. He isn't shot, his combat abilities aren't hampered at all, in fact he takes zero negative effects for being wounded until his HP drops from 1 to 0. To me that says that HP isn't the actual physical condition of your character, it's how much plot protection he has before the one ringer hits him that lays him out.
So a Cure spell or Breath of Life just restores HP, spells like Regenerate are needed to re-attach severed limbs and mend broken bones. But that's okay, because HP damage doesn't necessarily represent a punctured lung or a broken arm. Really you'd need some kind of penalty system. Like "If you take massive damage roll on the chart to see if you get your noggin cracked or an arm broken or something". Breath of Life could bring them back up, but it couldn't remove the penalty.
But that's a whole other system that would be a mess to implement.
As it is Sunrunnerii is right, RAW BoL would work fine on massive damage victims. I'm just sayin' if you fluff massive damage as getting torn in half or something you might want to consider that when making your call is all. Really it boils down to: As a GM how easy do you want it to be for your party to resurrect each other?