Bloomberg wouldn't serve in anyone's cabinet, especially not Trump's. They've had a good relationship in the past, but Bloomberg is very unimpressed by Trump's behavior and rhetoric.
No, Bloomberg was afraid Bernie Sanders would win. He dropped out as soon as Clinton took a significant delegate lead. He was always about stopping Sanders, not stopping Trump. The only thing he dislikes about Trump is that he might have lost to Sanders.
Bloomberg absolutely hates Donald Trump. Bloomberg is mostly liberal politically.
