I think he was simply conversing about a terrible thing that happened.
No he wasn't. He is running for President. Candidates for President do not simply converse. And he wasn't just lamenting on something terrible. He said the local police's handling was "terrible". So he was butting into a local police matter which is irrelevant to his mission to get elected President.
But what he could have said was that the justice department has no business getting involved in a local police matter. He could have used the issue to attack liberal politicians instead of a local police dept.