D&C 135

Merriam-Webster dictionary defines "testator" as -

"a person who dies leaving a will or testament in force"

The will or testament has no legal use until the testator dies. When it comes to the restoration of the gospel and covenants, it seems it was essential that the Prophet Joseph Smith was required to shed his own blood as evidence of the validity of revealed doctrine and covenants. Elder John Taylor wrote -

"TO seal the testimony of this book and the Book of Mormon, we announce the martyrdom of Joseph Smith the Prophet..." (Doctrine and Covenants Section 135:1)

He concluded his report that the Prophet died -

"...with the innocent blood of all the martyrs under the altar that John saw, will cry unto the Lord of Hosts till he avenges that blood on the earth." (Doctrine and Covenants Section 135:7)

Brigham Young later wrote -

"Many have marveled because of his death; but it was needful that he should seal his testimony with his blood, that he might be honored and the wicked might be condemned." (Doctrine and Covenants Section 136:39)

This was always the Lord's plan.