By ones who recognized that conflict would always arise between synthetics and organics.
The result is conflict, destruction, chaos.
It is inevitable
Shepard: Who designed it?
You would not know them, and there is not enough time to explain.
We first noted the concept for this device several cycles ago
The Crucible changed me, created new... possibilities. But I can’t make them happen (as in, crucible is not reaper device, but rather evolution of a device created by multiple cycles. the only solutions reapers ever had was harvest)
Synthesis.
Shepard: And that is?
Add your energy to the Crucible's.
The chain reaction will combine all synthetic and organic life into a new framework. A new...DNA.
Shepard (HOW?): Explain how my energy can be added to the Crucible.
Your organic energy, the essence of who and what you are, will be broken down and then dispersed.
(Cutscene shows a brilliant green blast)
Shepard: To do what exactly?
The energy of the Crucible, released in this way will alter the matrix of all organic life in the galaxy. ( pretty clear rewriting of dna here)