If you or I picked a fight with Gardner, got into a wrestling match with him, put him in a choak hold and he subsequently died of a heart attack, is there any doubt in your mind that we would have been indicted on some charge relating to his death?
But they didn’t pick a fight. They were enforcing the law. Gardner resisted arrest. Consider the scene in the movie Casino when the Feds were rounding up the mob members. The fat guy who kept the notebook with all of the details has an heart attack and drops dead. Did the Feds kill him? No. Did the stress of the event lead to his heart attack? Sure.
It appears in this case, though I am no expert, Gardner resisted, the police applied non-lethal force, however, due to preexisting conditions, he dropped dead. Is it a shame? Sure. Murder etc, nope.