Murder in the Desert

This is a story about three people (A, B and C) crossing a desert. A hated C and decided to kill him - he poisoned the water in his sack (only C had water). B also wanted to kill C (not knowing that the water of C had been already poisoned) and so B made a hole into the sack of C and the water spilt out. A few days later C died of thirst.
Who was the murderer - A or B?