Δ9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, is toxic for hippocampal neurons. Treatment of cultured neurons or hippocampal slices with THC caused shrinkage of neuronal cell bodies and nuclei as well as genomic DNA strand breaks, hallmarks of neuronal apoptosis
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6793471/
Apoptosis, also known as, programmed cell death
So, yeah, marijuana kills brain cells...