Apples to oranges. The WaPo dataset is behind a paywall so I cannot verify, but you used the term ‘police killings,’ whereas WaPo uses the term ‘police shootings.’ From previous interaction with the dataset I believe the WaPo source excludes non-firearm ‘police killings.’

You said yourself, suicide correlates with firearm ownership; I propose ‘police shootings’ likely correlates with the presence of firearms in police encounters which likely correlates to availability of firearms.

Furthermore, as somebody else mentioned, the time domain differs between the sets. You have different time periods, different classification criteria, potentially overlapping datapoints treated as distinct events. I do not believe any valid conclusions or observations could really be made from this presentation.