What could be the bad motive which makes a wrong act a morally bad act?
It reflects the absence of a certain sort of motive, namely, the desire to do right (or refrain from doing wrong). In fact, if a person does something which is not wrong but he does it in spite of his belief that it is wrong, he has acted immorally. This simply means that he acted in a way in which an immoral man would act. His manner of action shared a certain property with a morally bad action. That property was his indifference to the wrongness of the action. On the other hand, if a man did what was wrong but he believed it to be right and he did it from a desire to do the right thing, he would have acted morally. He would have acted in a way in which a moral man would have acted.
In short, what makes an act morally bad is that it is wrong and the agent is not deterred by its wrongness. When an act is very bad, morally, it is judged to be evil, atrocious, wicked, iniquitous, nefarious, fiendish, or demonic. “Wicked” applies to both the act and the agent. “Nefarious’ applies only to the act. “Heinous” expresses a strong disapproval of both act and the agent. “Vile” or “foul” expresses a strong feeling of disgust. What makes an act morally worse is the degree of the seriousness of the wrong done by the act. For example, it is more seriously wrong to murder a person than to steal a pen.