Genghis Khan the leader of the ruthless killers.
Ibn Kathir mentioned about his book that some historians mentioned that Ghenghis would climb a mountain again and again until he would feel Unconscious and he would order whatever i say in this state write it. Ibn Kathir said if this is true then obviously satan would speak on his tongue.[al Bidaya wal nihaya vol 13 under biography of Genghis Khan]
He also mentioned that mongols killed Muslims in bulk, raped the women in front of their husbands, many men died trying to save their wives and many were captured. Ibn Kathir also mentioned ruthless passage from his book and said all of this goes against Islam.
All of the above happened due to worst Muslim leader Khwarazm-Shah, when Genghis sent some businessmen who were taking the things of his country, a man killed them and Genghis sent a man to Khawarzam shah whether the businessmen were killed on his permission or not? Khawarzam ordered to kill Genghis’s man. Ibn Kathir said this was Khawarzam’s worst move and he was an idiot old man. Due to which Muslims suffered.
Note: They were not going to stop regardless of Khwarizm Shah act. They conquered even a major part of Europe and whole China. So their intention was clear.
Fun fact: Did you know it was Genghis Khan in the story of the Father, His Sons, and The Bundle of arrows.
Ibn Kathir mentioned from a narrator about the death of Genghis Khan.
ولما احتضر أوصى أولاده بالاتفاق وعدم الافتراق، وضرب لهم في لك الأمثال، وأحضر بين يديه نشابا وأخذ سهما أعطاه لواحد منهم فكسره، ثم أحضر حزمة ودفعها إليهم مجموعة فلم يطيقوا كسرها.
فقال: هذا مثلكم إذا اجتمعتم واتفقتم، وذلك مثلكم إذا انفردتم واختلفتم.
When his death was near he advised his children to stay together and not to be divided and mentioned some examples. He asked for an arrow and asked one of them to break it. And he broke it, then he asked them to break the full bundle but they were unable to break.
Genghis said: This is same if you remain together you will never be broken, and if you are alone you will be broken easily.[al Bidaya wal Nihaya vol 13 under his biography]