Death of the Family is a collection of comic book issues from many different series on the same event. When reading the full collection, that means each chapter (they refer to them as parts) features different protoganists fighting the Joker and the final chapter ties together all the stories.
While this pitch seems great, in practice, each chapter's story has the same shape. Something happens that springs a hero into action, the Joker uses the hero's predictable movement to put a counter measure in place, the Joker and the hero fight for awhile until eventually the Joker wins. I think the idea was to up the ante every time a hero is captured by the Joker, but the repetition makes it so predictable that I lost any sense of tension. Similarly, since the hero that is eventually captured in that chapter is also the protoganist, there is not another character who reacts with rage or distress due to their friends/family being taken by the Joker. It makes the terrible events feel less real to me since they are so infrequently referenced by the characters until the end of the story.
In, Death of the Family, Joker decides that the Robins, Bat Girl, etc (referred to as the Bat family). make him weaker, soft, and unfocused. Joker specifically thinks they are bringing him down cause Batman puts effort into caring for them and he wants Batman to focus on him. Batman overcomes in the end by trusting his Bat Family. Soooo I think the book is saying having family, friends, allies, etc. could make you weaker cause someone could target who you love to get to you, but if you trust them, you can be immensely stronger and actually take on challenges. I'm really stretching here to find that theme though and even if that is the book's theme, Spy Kids did it better.