What is your impression of Ozymandias as a king?
As described in the poem, Ozymandias was a tyrant who did not even make a pretense of caring about the needs of his people. His facial expression, his hand, his heart, and his voice all point to the fact that he was a selfish oppressor.
His facial expression was a "frown, and wrinkled lip, and sneer of cold command." This expression shows him as angry and unkind toward those he ruled over. A sneer is an expression showing contempt, meaning that he held his subjects to be inconsequential, not worth caring about, and deserving only of his scorn. The coldness of his command shows he had no empathy toward the plight of the people he ruled. He was self-absorbed.
His hand figuratively mocked his subjects. Again, this shows he scorned them. Whatever needs or wants they had were not to be entertained; his needs and wants were always supreme. To mock a powerful person can be considered brave and may have a purpose in bringing about change through satire, but to mock a needy or oppressed person is low indeed and displays a serious lack of morality. He was cruel.
His heart figuratively fed on his subjects. This shows he was a predator; he sucked the life from his subjects, perhaps requiring back-breaking labor for his building projects and the lion's share of anything they produced. That his heart fed on his subjects could also imply that he demanded their loyalty, praise, and adoration. Not being content with their obedient service, he demanded worship as well. He was abusive.
His voice, which we hear through the inscription on his pedestal, instructs the "Mighty" to look on his works and despair. This suggests that he sought to go beyond the bounds of his own kingdom—for no one in his own kingdom was mighty except him—to the surrounding region, contemplating conquest. The despair other kingdoms might feel would be inspired by their knowledge that, if he desired, he could subjugate them as well. Having everything at his disposal in his own land, he was not above desiring what lay beyond his borders. He was greedy.
From the evidence of his statue and inscription, Ozymandias was a self-absorbed, cruel, abusive, and greedy king.