This speculation is simply copying the hysterical media by seeking sensationalist news and making wild assumptions without any data.
Best wait for some facts.
Would speculation about racism be reasonable if the story had occurred in Apartheid South Africa? Why or why not?
The question you intended was "If there are details we do not know, is speculation productive?"
It is not.
As an aside, (i) it did not happen in Apartheid South Africa. (ii) I suppose that you would agree that not every death (Black or White) in police custody in Apartheid South Africa was as a result of police action.
I am basically very disappointed by the lack of forensic thought in this topic - it seems to have been overtaken by liberal "right-on" political correctness, which is not at all helpful when a potential prison sentence is at stake; it sounds like hysterical mob rule.