Each living thing reaches Nirvana when it faithfully follows the path set before it. A vegetable that grows finds Nirvana. They will fail in this quest if there is evil in their life, ie. A human may be reborn many times if they fail to follow the path faithfully. They could well bring upon themselves, in the next life, much suffering if they are unfaithful in this life.
Retribution is unmerciful.
Dharma - Corrupt Duty. Suadharma - Particular Duty. Caste - this works out to mean faithfulness to one's vocation in life. Ritual - Careful observance of the religious duties set before you. If a person has been faithful to the path before them, they become one with God.