When you deplore something, you disapprove of it very strongly and say so, often in public. Leaders deplore violence. A school may deplore cheating. Newspapers deplore a decision. It is a formal, public kind of verb — the language of statements, editorials, and speeches, not of a chat about a bad sandwich. Though actually, the older colour of the word is closer to mourning: to weep over, to lament. In modern news English that sadness is often thin, and deplore can sound like a stiff way to say “we condemn this”. You typically deplore an action, a situation, or a fact (“I deplore what happened”), more than a person — though angry speakers do sometimes deplore people too.