Speculation is that indeed Beria poisoned Stalin when he caught wind that Stalin was about to purge the entire Politburo.
The interesting story about how Beria was infamous for his pathetic kowtowing to Stalin, but hated Stalin. The story is that, while Stalin was in the process of dying, Beria was essentially berating the Big Dude. But Stalin awoke, and Beria faded before him, whining and crying. Pathetic. Two men who had no character.