Answer: Yes, you should immediately have a confidential talk with your HR manager. Your colleague’s devious behavior is both appalling and a little frightening.
His suspiciousness, combined with a growing “enemies list,” may indicate paranoid tendencies. If he feels betrayed, he could decide to seek revenge. So when you report his clandestine activities, ask the HR manager not to reveal your name.
If you fear that HR may fail to protect your identity, describe the situation in an anonymous note. Although unsigned complaints are often disregarded, is unlikely to ignore this one.