Not talking about What If Killmonger, but the live action Killmonger.
Of course, being a raving manchild surely prevents someone from being magificent enough because they are too immature and doesn't go hand to hand with their magnificence. And so is xenophobia because he's also too broken by his hatred.
But what makes him so vile is that he killed his girlfriend and intends on killing everybody else that he thinks is a threat to him for no reason other than spite. However, it's up to you if he still has the qualities that prevent him from being too vile.