According to Tom, what is man's deadliest enemy?

According to Tom, a man's own hand is his deadliest enemy.

"Caesar's ghost!" commented Tom, with astonishment. "It beats anything that was ever heard of! Why, a man's own hand is his deadliest enemy! Just think of that—a man's own hand keeps a record of the deepest and fatalest secrets of his life, and is treacherously ready to expose himself to any black-magic stranger that comes along.