Gevurah is the concept of ‘judgment’ or ‘restriction.’ Something that ‘restricts’ the light of G-d would be a fundamental definition of ‘evil.’ Thus Gevurah, in the middle of the left side of the Tree of Life, is seen as the ‘source’ of evil.
Sin acts in a restrictive way if we let it. We do wrong and begin to think of ourselves as a negative manner, distancing ourselves from G-d and his mercy, rather that seeking to do teshuvah – returning to our true self, ‘made in His Image.’
We may thus be inclined to view Gevurah as ‘negative,’ but as with all the middot, it is really neutral. Its application determines if it is ‘good or bad.’
For instance, a woman from the Tenach associated with Gevurah was Sarah, whose correct judgment caused Hagar and Ishmael to be removed from the picture. The numerical value of שרה (Sarah) is the same as דין אמת , “true justice.”