Wikipedia says that BLM founders are:
Alicia Garza - alive and well
Patrisse Cullors - alive and well
Opal Tometi - alive and well
Under "key people" - Shaun King, DeRay Mckesson, Johnetta Elzie, Tef Poe, Erica Garner. Of those, only Erica Garner is not alive - she died in 2017 of two heart attacks and the following brain damage.
BLM is a decentralized movement, so there are a lot of people that could be called "leaders", but clearly claim that "nearly every leader" has been killed is false. I suspect none were actually killed for any activities related to BLM (I did not check that but consider it plausible), though in large enough group of people there would always be people that die prematurely, either due to illness, or accidents, or crime (many BLM activists live in places where crime situation is not very good).
I think you'd better to put down the conspiracy Kool-Aid for a while.