If Loki's tears are any standard to go by, then it's just the tip of the iceberg to what a villainous character is able to cry, see: Erik Lehnsherr. Not even funny.
Loki sheds tears twice, in highly emotional moments that WARRANT it. Also only in the presence of first his father, later his brother. Truely, Loki only makes innocent faces most of the time where you feel the undercurrent of schemes beneath his gaze. With Erik? Not so. He just cries at every given opportunity to show how conflicted he is. Doesn't matter who sees it Dx OR the situation *kicks and screams*