tl;dr: First, read it. Second, Reaper’s a mass of nanobots, and I’m sticking to it.
It would also explain the following exchanges:
Mercy: What happened to you?
Reaper: You tell me, Doc.
especially this one:
Mercy: This is not what I intended for you, Reyes.
Reaper: You knew exactly what were you doing.
Since Reaper seems to blame Mercy who is apparently a premier researcher in the field of applied nanobiology.