The case of Satish Shrivastava

Satish Shrivastava was an old , invalid and immobile man. He was once a successful chemist and had a lot of patents and discoveries to his credit. But a paralytic attack ruined him and made him immobile. However his hands were reasonably healthy. He had been provided with a manservant Ramu by his past research organization to take care of him. Ramu had Down's syndrome and had little intelligence. The residence of Satish Shrivastava had webcam monitored by Able Security Agency. Gaurav Patekar was the president of Able Security Agency.
Satish Shrivastava was found poisoned on a Monday and he had died. He was found dead at 8:30 pm. Poison had entered into his body at 7:30 pm.
Inspector Daniel Paul started his investigations. He started by looking at webcam footage of the fateful evening. Unfortunately for some seemingly suspicious reason webcam footage from 7 pm to 8:30 pm was not available. The footage was just dark. The 6:30 pm footage showed Ramu asking the permission of Satish Shrivastava to go to market and buy rations.  Satish Shrivastava asks Ramu to keep the bottle of water nearer and gives permission to leave. The 8:30 pm footage showed Ramu crying near his master's dead body. Inspector Daniel Paul discovers the poison was in the bottle of water near Satish Shrivastava.
Who killed Satish Shrivastava ?