The Blood Golem distinguishes itself from other golems in that it links the Necromancers life to its own. If the golem takes damage, so does its master. The Necromancer can never die from this link, but hp can be lowered quite low.
Of the life stolen, 70% goes to the golem and 30% goes to the necromancer, meaning the golem can also heal his master. If any of the two are at full hp, the overflow is transferred. So if the Blood Golem has 100% life, all the life he steals will be given directly to the Necromancer.
Utilizing a small quantity of his own blood, the Necromancer is able to give life to a creature neither living nor dead, yet formed of human tissue. This homunculus gains nourishment from the fresh blood of its slain enemies, from which it replenishes its strength. Tied to the Necromancer through the mage's own blood, the Blood Golem is able to share this stolen life force with his master. Unfortunately, the link flows both ways, and any damage taken by the Golem is also transmitted to the caster.
↑Diablo II Manual, page 76. Blizzard Entertainment, 2000.