The rebel leader declared himself king and was seen as a usurper by the people.

usurper 是动词 usurp（篡夺、夺取）加后缀 -er 构成的名词，表示"进行篡夺行为的人"。usurp 来自拉丁语，原意是"非法夺取权力或地位"。 在句中指那个自封为王的叛军首领，因为他是通过非法手段夺取王位的人。