He got terrified. He was afraid that Julia would break down and tell, so he went to Julia and killed her and left Wynant's watch chain in her hand.
12 seconds sound clip from the The Thin Man (1936) classic radio play.
You can hear this line at 00:49:34 in the radio play.
Quote context
[...]
- But it must have been Wynant. I should have known if it weren't his voice.
- Oh, he was clever about that. He called when you were out.
- Is that true? I don't know. Why are you saying it? It's the only way it makes sense.
- His plan was still working beautifully.
- The only hitch was a man named Nunheim who had found out something.
- Our hero bumped him off too.
[...]