Health Tag doesnt work, or at least is not longer in minecraft
(I think it was probably removed at 13w16adue to maxHealth Attribute)
But the only official information we have it is at minecraft wiki
http://minecraft.gamepedia.com/Chunk_format
But i asked them and they said that it hasnt changed, SO, health should be bugged. (or deleted, but we have no information about that)

This ''bug'' its easy to test with horses (horse Health bar), but you can check that a diamond sword wont kill a 2 health (1 heart) villager, Health tag doesnt work.