"Every parent knows that the child's provocations, wild and "transgressive" as they may appear, ultimately conceal and express a demand, addressed at the figure of authority, to set a firm limit, to draw a line which means "This far and no further!", thus enabling the child to achieve a clear mapping of what is possible and what is not possible. [...] Paradoxically, it is the setting of a firm limit which is liberating, and it is the very absence of a firm limit which is experienced as suffocating. THIS is why the Kantian autonomy of the subject is so difficult - its implication is precisely that there is nobody outside, no external agent of "natural authority", who can do the job for me and set me my limit, that I myself have to pose a limit to my natural "unruliness."
Although Kant famously wrote that man is an animal which needs a master, this should not deceive us: what Kant aims at is not the philosophical commonplace according to which, in contrast to animals whose behavioral patterns are grounded in their inherited instincts, man lacks such firm coordinates which, therefore, have to be imposed on him from the outside, through a cultural authority; Kant's true aim is rather to point out how the very need of an external master is a deceptive lure: man needs a master in order to conceal from himself the deadlock of his own difficult freedom and self-responsibility. In this precise sense, a truly enlightened "mature" human being is a subject who no longer needs a master, who can fully assume the heavy burden of defining his own limitations. This basic Kantian (and also Hegelian) lesson was put very clearly by Chesterton: "Every act of will is an act of self-limitation. To desire action is to desire limitation. In that sense every act is an act of self-sacrifice."
Clark isn't brooding at the start of the film, he is portrayed as quiet and literally sensitive, even effeminate - which the bullies (and his coworkers on the boat, and the drunk dude, Faora, etc.), pick up on. He stays quiet because it's easier for him, because he's afraid, and because he needs guidance to become a fully-realized human being before he himself can lead mankind. The violence at the end of the film isn't facile and simple at all, but incredibly burdensome, culminating in the painful killing of Zod - painful because Superman is fully confronted with the terrifying responsibility of being free.