Losers are always in the wrong.

解释 ( paraphrase )
旧指在争夺政权斗争中，成功了的就是合法的，称帝称王；失败了的就是非法的，被称为寇贼。
这条谚语含有成功者权势在手，无人敢责难，失败者却有口难辩的意思。

That one will become a king or an outlaw depends on whether he is successful or not. Legitimacy belongs to the victor.
This proverb indicates that the winner takes complete control of authority that no one can challenge; the loser finds it hard to vindicate himself.

寇：强盗或外来侵略者。
      Bandit or invader.

造句 ( example )
说话有理的人好像打了胜仗的大王，神采飞扬；而争执不过的人就好像被俘虏的士兵，垂头丧气。这就像“成者为王，败者为寇”。