Why did Uthman ibn Affan not kill Ubayd Allah ibn Umar when he killed Al-Hurmuzan and the daughter of Abu Lu'lu'ah al-Majusi?

The scholars have provided several answers regarding Uthman's pardon of Ubaydullah ibn Umar for killing Al-Hurmuzan. Among the best of these are two answers mentioned by Ibn Taymiyyah: The first is that Ubaydullah believed Al-Hurmuzan assisted in the killing of his father, and this was a doubt (shubha) that prevented the obligation of Qisas (retribution in kind). The second is that Al-Hurmuzan had no guardians (awliya') to demand his blood; rather, his guardian was the ruler, so Uthman chose blood money (diya) over Qisas. This latter answer also explains Uthman's pardon of Ubaydullah for killing Abu Lu'lu'ah's daughter, as Uthman ransomed those slain individuals from his own wealth because their affair was entrusted to him and they had no heir except the public treasury (Bayt al-Mal).