Jehovah's Witnesses have officially ruined my LIFE...

Why do they refuse blood transfusion? I saw a film where this happened and the girl died.

Most strict Jehovah's Witnesses refuse all direct blood transfusions and the primary components derived from other people's blood donations (including red cells, platelets and plasma).
JW theology does not allow any distinction to be made between voluntarily consuming blood within a food or fun context and accepting donated blood into the blood vessels as part of a lifesaving medical procedure.

JW elders preach that God commands human believers to abstain from consuming all blood with multiple verses within the Old & New Testaments.
For example: Acts 15:28-29, Deuteronomy 12:23, and Leviticus 17:10.
Any JWs who accept a lifesaving blood transfusion (either for themselves, their child or an adult relative) are harshly punished via disfellowshipping and become a social outcast that all practicing JWs are forbidden to have any contact or interaction with.
Same applies to any JW who recommends a blood transplant to another JW or a next of kin does not object to doctors performing a lifesaving transfusion without giving medics any formal consent.

I don't agree with blood transfusions and would never agree to accept one, I am not a JW.
I have seen the appalling tactics and revolting harassment that teams of JW zealots frequently resort to within hospital premises attempting to prevent very ill random strangers from accepting blood transfusions.
Targeting non-JWs and JWs alike.
The hospital teams are often volunteers composed of the most bad mannered & extreme of door knocking conversion groups.
They have not been invited to attend the hospital and have no business entering the wards nor bothering any patients.