Of course this divine weapon called the Blood Sacrifice divine ability needs to be polished and prepared. To do this, they need to kill a lot of people and siphon a lot of life energy.
The more life energy they use to polish and prepare the Blood Sacrifice divine ability, the more powerful it will become. So in a way, the Blood Sacrifice divine ability is a method to kill stronger foes by killing a lot of weaker foes first.
The Blood Sacrifice itself is a fast ranged attack in the form of a blood-red lightning bolt. This is great because killers don't have many means for ranged attack.
The Blood Sacrifice solves a dire need for ranged attack and is also blindly fast. But the best part of it is that as complete angels, they can enhance the blood-red lightning bolt with other divine abilities in their pathway to strengthen it further.
