The police and the mother are both in the right.
Could the police have used a taser instead of a fire arm? I wouldn't be surprised to find out yes, it could have been used and nobody would have died THAT night.
Yet, if you are threatening the police after assaulting two other people earlier in the evening, the best thing you can do is drop your weapon and assume the position. I wouldn't count on getting tased over lead poisoning. To ask cops to use less lethal force in the face of lethal force is just inane. YMMV