ruthless       
adjective uk/ˈruːθ.ləs/ us/ˈruːθ.ləs/
C2 无情的;冷酷的;残忍的
not thinking or worrying about any pain caused to others; cruel
  ruthless ambition 冷酷无情的野心 
 a ruthless dictator 残忍的独裁者 
 Some people believe that to succeed in this world you have to be ruthless. 有些人认为,在这个世界上要想成功必须冷酷无情。