The Sefirah of Gevurah is called Strength only by virtue of the power and the greatness of the previous Sefirah of Chesed, which has to be confined and channeled within definite borders in order for it to have any effect.
The strength of Gevurah is essentially a function of pointedness, of force withheld, directed, and focussed at will.
As it is said: Who is the hero—he who controls his impulses.
The dam halting the flow and compelling it to move in a desired direction is the essence of the power that is Gevurah.
In any case, these two Sefirot — Chesed and Gevurah — are basic forces in life, one flowing out of the self and the other withdrawing into the self, controlling and restraining.
Each can be used either for good or bad.
For example, one of the distortions of Chesed is the sin of fornication, while the Sefirah of Gevurah or Fear (of God) can also be expressed in a variety of questionable ways.
Thus, the Tzadik of Lublin once said that when a person falls from the level of Chesed he becomes a lecher; when a person falls from the level of Gevurah, he becomes a murderer.
There is also the matter of basic temperament, of course, something a person does not easily change; one can, at best, alter the expression, for good or bad, of temperament or character.
Thus, the same person who is constitutionally of a Chesed type may grow in goodness to become a "Chesed Avraham" character, or he may become a profligate 'Don Juan" personality.
–Rabbi Adin Steinsaltz
From The Sustaining Utterance, "Concealment as Part of Creation," p.45 by Rabbi Adin Steinsaltz