What is the evidence from the Qur’an and Sunnah that major sins are despicable?

We say that sins are trivial compared to Allah's forgiveness for the repentant, so that they do not despair, in accordance with His saying, the Exalted: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" As for the one who persists in sin, he is told: "All sins are great, and do not look at the smallness of the sin, but look at the greatness of the One you defied with it." The remedy must be applied to the ailment, and each addressed with the appropriate discourse. The importance of sin should not be diminished in one's soul; rather, one must be mindful of Allah's awareness when contemplating it. If a sin occurs, one must hasten to repent, combining fear and hope, in accordance with His saying, the Exalted: "Inform My servants that it is I who am the Forgiving, the Merciful. And that it is My punishment which is the painful punishment."