1: The mother needs a definable goal.
2: Mass shootings?are usually stopped by the police, In other words, the shooter stops killing when the police arrive and kill or apprehend the killer. This makes the police the good guys.
But in your story, the police seem to be the bad guys.
Therefore, if you want the police to be the main antagonists, you should have the husband shot by the police, not shot in a mass shooting. This is to avoid dividing the focus.
As for the story, Yes I think, if you give the lead character a definable goal and keep the focus tight, it could make a very topical mini-series.
Here would be an example:
"When her husband is killed by the police, who mistake him for an active shooter; an inner-city mother seeks justice in a city where opposing the police has deadly consequences."
-----
Perhaps the husband tries to stop a mass shooting and wrestles the gun away from the shooter, the police bust in, see him with the gun and shoot him. That way you could keep the mass shooting angle but the husband would still be wrongly shot by the police.