In the Hadith, Mohammed says that the Mahdi will "fill the Earth with justice as it has been filled with corruption."
"Hadhrat Abu Saeed Khudri (R.A.) compares that Rasulullah (S.A.W) said, 'Al Mahdi will be from my progeny. His forehead will be broad and his nose will be high. He will fill the world with justice and fairness at a time when the world will be filled with oppression. He will rule for seven years.'"
[some say seventy years --Dajjal]
"Other hadeeth tell us the description!, of Imam Mahdi (A.S): -He will be tall -He will be fair complexioned -His facial features will be similar to those of Rasulullah (Sallallahu Alayhi Wasallam) -His character will be exactly like that of Rasulullah (Sallallahu Alayhi Wasallam) -His father's name will be Abdullah -His mother's name will be A'amina -He will speak with a slight stutter and occasionally this stutter will frustrate him causing him to hit his hand upon his thigh. -His age at the time of his emergence will be forty years -He will receive Knowledge from Allah. -His Emergence and Rule"
For the Sufis and Ba'hai, the Mahdi will not kill his opponents, but through peaceful yet powerful preaching will "kill" the deceitfulness and infidelism in the Dajjal and his followers, convincing them to accept Islamic shar'ia.
Frankly, I'm growin' kinda fond of the Sufis and Ba'hai.
Very unfortunately, so little is available in English about Wahabi theology, that I cannot say for certain how the Wahabis picture the Mahdi; though I am willing to make some wild guesses.