In her new thriller, Cruel Justice, Mel Comley introduced me to the expression the cat that got the canary. This extremely evocative expression puts me in mind of Sylvester who, after long years of torment, finally manages to swallow that damn bird. In the context where Mel uses it, someone is feeling very pleased with themselves. An expression exists for this, and it is: the cat that got the cream.
This is an expression that has existed for a very long time indeed, and is an expression of smugness or self-satisfaction, as it describes the expression of the first cat at the churn, who gets to drink the cream on the top of the milk before the other cats arrive and have to make do with the milk.
This isn't quite what Mel was going for, though I think it would have fit. The cat that got the canary is somehow … predatory.