Herschel believed what separated humanity from other life on Earth was it's capacity for higher intellectual acuity coupled with compassion.
Sure, other animals demonstrated intelligence and compassion, but not the ability to exercise both simultaneously.
Yet people also were capable of great cruelty and indifference; a trait, he felt, not shared with other animals. Animals acted instinctually, people more purposely. That indifference allowed people to forget unpleasantness and move on.
That's why he believed the authorities would never discover what he had done. They would express initial outrage, pursue their investigation with zeal, but eventually give up and move on.