Good and Evil are both part of the Whole, the Whole is already Whole, you don't need to make it so, you just have to accept it within your own psychology.
Problems in the world are not from evil, they are from dualism, dualism leads to the real problem which is a state called 'Soul Sickness', Soul Sickness is something like OCD, a pathology where you think you will only be whole if you negate what you don't want, but in reality if you stop negating what you don't prefer then you will more easily be able to move towards what you do prefer because you won't be weighed down by the attachment of negating what you don't prefer, so you can then walk around with a lightness of being, a sort of ecstatic monism.
You can ask yourself in any given moment if you are being ecstatic or retractive, to be retractive is to be soul sick, you are still negating what you don't prefer, and therefore you are heavy, in order to move forward you have to embody a lightness of being, you simply recognize that all things; good and evil are already part of the whole, but like I said you can still move towards your preferences by not negating what you don't prefer.
You are all part of the monad, a super-essential oneness or positive vacuum, you are individuated, but that individuation is not separateness. So the real issue here is not Evil, instead it is the neurotic pathology of Soul Sickness which is the product of Dualism and Fear. What you really want to do is to be ecstatic by day, and chthonic by night.
Evil is not what you think it is, it's a valid quality and useful quality, by negating it you are denying your Sinisterial and Feminine power which includes thing like sexuality by the way. It's not Evil that is the problem, instead it is the neurotic pathology of soul sickness, mental imbalance, OCD, carrying a mental heaviness as opposed to an ecstatic embodied lightness of being.
This answers the false dilemma of why evil tends to win; because evil is just an ecstatic sinisterial flow, whereas to be 'good' in a dualistic sense you are always at neurotic odds with yourself, this is the problem of the Abrahamic hyper-alertness or neurosis, and other variegations of Logocentrism that Ludwig Klages warned of.