Why does Nick refuse to shake hands with Tom?

Nick refused to shake hands with Tom because of what he'd told George Wilson. Nick knew that whatever Tom told Wilson caused him to murder Gatsby.

“What’s the matter, Nick? Do you object to shaking hands with me?”

“Yes. You know what I think of you.”

“You’re crazy, Nick,” he said quickly. “Crazy as hell. I don’t know what’s the matter with you.”

“Tom,” I inquired, “what did you say to Wilson that afternoon?” He stared at me without a word, and I knew I had guessed right about those missing hours. I started to turn away, but he took a step after me and grabbed my arm.