It is widely popular that the Basmala (or as we know it as Bismillah) does not appear before Surah Tawbah because of the surah being that of anger. However, the reality is not as we widely popular. Mufti Mahmood Hasan rahimahullah was asked about this and he replied as follows:
A: When Hadhrat Uthman radiallahu anhu was asked why there was no bismillah at the beginning of Surah Taubah in the manner that there is bismillah at the beginning of other surahs, he replied that there was some doubt as to this surah being a separate surah, because Nabi salallahu alayhi wasallam passed away without explicitly mentioning anything regarding it.
This is why Bismillah was not written at the beginning of Surah Taubah. However, since its subject matter is similar to that of Surah Anfaal, it was placed after it and a space for Bismillah has been kept because it is possible that this is a separate surah. (Tirmizi, Vol 2, Pg.139) 
In Bukhaari Shareef it is mentioned that this surah was revealed to cancel the peace treaty. In it appears the command of killing the mushrikeen (polytheists) and the expression of Allah Ta`ala’s anger over them.
Therefore, Bismillah was not written at the beginning of the surah since Bismillah signifies peace and mercy.
It appears in Shaatbi:
(Bismillah does not appear at the beginning of this surah, because it was revealed in connection to war.) 
This is not the actual reason though it is a point to consider. The actual reason is that which is narrated from Hadhrat Uthman radiallahu anhu (Bayaan-ul-Qur’aan, Vol 4, Pg 95)