Taking Dead Cat with 1 or 2 Heart Containers won't kill Isaac because it will set his number of Heart Containers to 1 immediately, saving him from the death that would have resulted from having no Heart Containers. Buying Dead Cat when at 1 Heart Container essentually obtains the item for free.
Other Devil Room items can be bought without the necessary number of Heart Containers at the cost of one of the many lives Dead Cat provides.
Taking Dead Cat as Keeper or Tainted Keeper will reduce his total number of Coin Hearts to one, meaning he will die upon taking any damage at all. Unless Keeper has immediate access to an HP Up item, it is not recommended to take Dead Cat as the Keeper.
Additional Dead Cats can be picked up during a run, increasing lives well past 9. Only the first dead cat counts toward becoming Guppy.
This item counts as 9 items, so if Isaac is trying for an early portal to after beating Mom, this will significantly boost the progress closer to 20 items.
If Jacob and Esau take the item, only the brother who takes it will be set to one Red Heart Container on pickup. But if they die, both are resurrected with one Red Heart Container.
If Tainted Jacob takes this item after being touched by Dark Esau, he will return to his previous Red Heart total upon reaching the next floor.
Dying as Tainted Jacob after being touched by Dark Esau and carrying this item will respawn him in the previous room in his lost form. Making it to the next floor will still return him to his previous Red Heart total.