Shia al-Mahdi will be mass murderer.

Allah said in the Quran (surah al-Israh, 33):

Nor take life – which Allah has made sacred – except for just cause. And if anyone is slain wrongfully, we have given his heir authority (to demand qisas or to forgive): but let him nor exceed bounds in the matter of taking life; for he is helped (by the Law).

Shia Quran commentator narrated in his Tafsir al-Ayashi (2/290):

67 – عن سلام بن المستنير عن أبى جعفر عليه السلام في قوله: (ومن قتل مظلوما فقد جعلنا لوليه سلطانا فلا يسرف في القتل انه كان منصورا) قال: هو الحسين بن على عليه السلام قتل مظلوما ونحن اولياؤه، والقائم منا إذا قام منا طلب بثار الحسين، فيقتل حتى يقال قد أسرف في القتل، وقال:  المسى

From Salaam bin al-Mustanir from Abu Jafar (alaihi salam) that he said: “It was Husain bin Ali (alaihuma salam) that was martyred unjustly and we are his successors (friends). When the Qaim from us rises he will set out to revenge the blood of Husain (alaihi salam). Then he will unleash such killings that it would be said that he is doing Israaf (excess) in killing…”