This man is among the reasons that the Ukrainians did not do the investigation. He told them not to.
A lot of the “witnesses” have dug their own (metaphorical) graves.
They’re basically testifying and even boasting that they tried to subvert US foreign policy, which is determined by the President, like him or not,
and overrode Trump’s orders through their prior Obama era connections. And they also seem to have assured some of these foreign leaders that Trump wasn’t really in charge, and they’d make sure the same old same old continued and “take care of them” (NY expression for slipping a few bucks into somebody’s hand).