Fires, hurricanes, extreme heat, and drought are made more frequent & severe by climate change, and they certainly kill people. Climate change also disrupts food systems and will cause civil war and mass migrations. Millions will need to migrate just from coastal flooding, which I'd imagine will cause plenty of geopolitical strife and ultimately lead to many deaths.

"Existential threat" in my mind means humanity itself is threatened. The species will likely survive but millions if not billions will die due to climate change.