The old mosaic law says that blood is sacred because it represents life. Because of this. people could kill animals for food but they were commanded to drain the blood first.
The religious leaders of Jehovah's Witnesses decided that blood transfusions are the same as eating blood. They argue that if a doctor told someone to abstain from alcohol, does that mean a person could inject it directly into their veins?
According to the secret book for Elders only, if a JW has a blood transfusion and sees nothing wrong with it, they 'disassociate' themselves from the organization.
Persons who leave the organization and disagree with some or all of the teachings of the Witnesses are shunned by family and friends (with limited exceptions). They are treated the same as a 'wicked' person who gets 'disfellowshipped' for 'wrong doing'. See Reasons for Disfellowshipping
As a result of the church's teachings about blood transfusions, zealous Witnesses would rather die than have a transfusion.