Apple, Google, Facebook and Amazon stock all ended the day in the red with Amazon down 2%. Trump has specifically claimed that "Amazon is getting away with murder tax-wise," and could try to take action to change that.
It doesn't divide like that. The idiom is "getting away with murder", which isn't literally about murder, but about 'getting away with' (= not being caught or punished for) things.
So he means that they're doing something wrong about their taxes, and not being called, or caught, or punished, for it.