The ethical problems with the Krogan stem from their massive birth rate: an adult female who lives for centuries pops out ~1 surviving child per Solar year and ~999 that don't survive, or a total fertility rate (TFR) in the triple digits. After the Rachni Wars, the Krogan population exploded and they temporarily became an expansionist empire until the Turians unleashed the genophage.
However, this explanation runs into several inconsistencies and logistical problems.
- Their total fertility rate accounting for child mortality would be in the triple digits; for comparison, the highest human TFR is 6.89 in Niger. Someone needs to raise all those offspring and teach how to speak and use tools and so forth. Communal child rearing could never account for that many offspring.
- The writing conflicts on their biology: Krogans are said to both lay eggs and experience pregnancies at different points. The numbers listed above make those options physically impractical and would require mass spawning like fish or amphibians.
- The game uses two conflicting social portrayals of the krogan depending on whether it is arguing for or against the genophage, even though the two portrayals are irreconcilable and their rate of population growth is simply impossible without a hive-like caste system like the Rachni or Salarians.
- Within the story itself outside the Codex, the Krogan are portrayed raising a small number of children exactly like humans rather than the dozens you would expect; presumably so players would sympathize despite the absurd numbers listed above.
I could accept an explanation where the Salarians used their tech to raise massive armies that would otherwise have died in childhood; resulting in a massive imbalance in the krogan population which, combined with the extreme poverty of their home world, led to a rise in nationalism and subsequent imperialism. But a TFR in the triple digits is blatantly absurd, ruins my suspension of disbelief and invalidates the dilemma because obviously curing the genophage would be out of the question.