As in what part (or parts) of a character that are terrifying in action or given thought. It can be looks, personality, ideology, and/or an ability if applied with real-life implications.
For instance, Doomfist is scary because of his ideology; the concept of using conflict to further humanity implies a never-ending wave of war where if you aren’t strong, you will suffer or die.
Moira basically just wants to experiment on humans so that’s not great, to be in the same boat ideologically as Mengele.
Well at least she’s not sewing kids together