If a man murders a woman, then according to ‘Qisās’ (as I understand) -"a woman for a woman" should be killed. But why should an innocent woman be killed? Why not the murderer himself? Please explain.

If the referred Qur’ānic verse is translated properly, your question does not arise. Verse 178 of Sūrah Baqarah reads thus:

O ye who believe, decreed for you is the qisās of those among you who are killed such that if the murderer is a free-man, then he [himself] will be executed in lieu of [the crime] and if he is a slave, then he [himself] in lieu [of the crime] and if she is a woman then she [herself] in lieu [of the crime].

In other words, what the verses actually imply is that it is the murderer only who should pay the penalty for the crime. No one else should be killed is his or her place. It should be kept in consideration that in the pre-Islamic Arab society, there were customs in which murderers were let off if they belonged to a certain tribe or class of society. In their place, some person of an inferior creed was executed. Sometimes, a slave was executed for the crimes of his master. At other times, simply no one was executed. This verse actually stresses total equality in matters of qisās. It is the murderer himself or herself who must be penalized for the crime he or she has committed. Consequently, `a woman for a woman' does not at all mean that if a man has murdered a woman, then in his place a woman should be executed. The man himself must be put to death.