Thats a good point that injury could have driven Maegor even more over the edge but this is a guy that mocked his own King's power darign him to even try and take blackfyre and had no qualm about even torturing his nephew although he knew he didn't know where alyssa went. I mean they do hint his cruelty was exaggerated, but with how he wiped out the Horrways and killed Tyanna. along with holding the grand maesters head for comfort yea Cruel seems an understatement. Still as many said he was shown to at least value people at first the world book said he took his mother's death in strides, but we see him making it a point to make sure she joined her brother and sisters ashes. Did actually listen to her counsel as well.
Man who would have thought Blackfyre would play a role for choosing a king again even before the blackfyre rebellion. Why would Visenya of all people think Aenys giving Maegor the sword proved he was fit to rule.