President Trump said in his address yesterday, “George Floyd will not have died in vain” as you see in the following excerpt.
“Trump vows ‘George Floyd will not have died in vain’ as National Guard arrives in Minneapolis. The president urged residents of Minneapolis to honor Floyd's memory, assuring them that his death will not be in vain.” (disrn.com)
Is the future perfect grammatically correct in this context when George Floyd is already dead?
I agree with Kinto that using the future perfect here is grammatically correct.
It simply means that Trump is going to do something concerning this issue at some point in the future. After this point, this man's death or the reasons behind his death will no longer be in vain.