The villain in the movie ended up taking his own life at the end.
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Literary / formal - common in historical fiction, wuxia novels, news reports about political figures. The components break down as 自行 ('on one's own') + 了断 ('to settle/end definitively'). More everyday Chinese says 自杀 (suicide).
mistakes
了 here is read liǎo (to settle, conclude), not the common particle 'le'. Saying zìxíng-le-duàn is wrong - it's zìxíng-liǎo-duàn.