According to BahaiAwareness,  probably the most well-known anti-Bahá’i site, the Báb and His disciples were violent. Here are a few quoted passages used to support this assertion:

The Babi Shariah directs that all who did not accept this Shariah should be put to death,
(Ref: Makatib, Volume 2, page 266)

O my followers! Surely Allah has made fighting obligatory for you. You must conquer the cities and the people for Babism and don’t be at peace with those who reject Babism”.
(Ref: Bayan, Arabic Chapter 1)

It is the duty of every Babi king that he should not allow any non-Babi to live n his realm. This is also binding upon the other Babis as well. The only non-Babi who could be allowed to live in the country was one who carried on some trade of benefit for other people.
(Ref: Bayan, Bab 4, Wahid 8)

The author concludes with the following statement:

Needless to say that these beliefs of the Bab are in stark contrast with the beliefs of the Holy Prophet of Islam (pbuh) who propagated peace in the true sense, who bade us to do good to our parents even if they be non-Muslims, to live peacefully with our neighbors and to maintain their rights.

Now, what if I told you that the Mahdi is prophesied to be violent? Ironic, huh? Let’s see if this is true…

Zurarah asked Imam as-Sadiq (‘a): “Will the approach and policy of Hadrat al-Qa’im (‘atfs) be the same as that of the Prophet (S)?” The Imam (‘a) replied: “O Zurarah! Never; never. He will not adopt the policy of the Prophet (S) (in dealing with enemies). The Prophet (S) used to deal with enemies leniently, gently and kindly in order to win their hearts and for the people to be acquainted with him. Hadrat al-Qa’im (‘atfs), however, will adopt the policy of killing; he will act according to his order and accept no one’s repentance. Thus, woe be to anyone who would oppose him!” (An Overview of the Mahdi’s Government)

Imam Baqir (‘a) said: “Hadrat al-Mahdi (‘atfs) will act in such a way that most of the people will say, ‘This person is not from the Progeny of Muhammad (S) and if he were really from the Ahl al-Bayt of the Prophet (S), he would have been compassionate’.” (Ibid.)

So the article from BahaiAwareness argues that the Báb can’t be the Mahdi because He and His disciples were violent while they, Twelvers, expect Him to be violent. Just one more reason why the Báb is Who He says He is.