Sri Krishna is talking about the maturity of the mind through which one can understand what one’s dharma is and how one can perform the karma associated with it. He has been telling Arjuna how to control the mind and direct his actions.
The law of creation is such that it allows the positive and the negative to manifest, but it will not allow the negative to rule; it allows the positive to rule. The negative can be strong, but it is not permanent. The positive can be weak, but ultimately it will rule, it will win, it will dominate.
Sri Krishna sees the negative and the positive, not just in an individual, but in nature also, which is part of the manifest creation. If the negative forces were to win in the end, it would defy all the laws of evolution, of progression and all laws of divinity.
God represents the sublime, the good. God-consciousness, higher consciousness, is pure, transcendental and divine and if everyone attempts to attain that state, then the negative is only a transitory phase. Therefore, it has been said, Satyam eva jayate – Truth alone triumphs.