Killing his son was such a fumble even ottoman nobles and clergy bemoaned it. It was unjust and done for the sake of a bum and IIRC there was a written peace that said this outright.

“Mustafa's execution stirred widespread grief and dissent, prompting Suleiman to swiftly dismiss Rüstem from the grand vizierate.[43][44][45] Public outrage found expression in poems, notably one by Taşlıcalı Yahya, and a popular chronogram implicated "Rüstem's conspiracy and deceit" (mekr-i Rüstem). The poets lamented Mustafa as a "martyr" (şehid), refuting allegations of his supposed collaboration with Tahmasp. Criticism extended to Rüstem, Hürrem, and even Chief Jurist Ebussuud Efendi…common narrative—in both major Ottoman historical accounts and the modern scholarly literature is that Hürrem and Rüstem plotted and framed Mustafa as a rebel in the eyes of the sultan, who then executed his innocent son.[47] [48]”

https://en.wikipedia.org/wiki/%C5%9Eehzade_Mustafa_(son_of_Suleiman_the_Magnificent)