It’s not who appointed the guy in charge of the IRS during this time it’s under whose watch it happened. The IRS was not used against liberal groups under Bush it was used against Conservatives to suppress the vote under Obama.
Also, the head of the IRS who was appointed by Bush was a Dem, because Bush was devoted to the “non-partisan” ideal and bent over backwards to show that he wasn’t partisan. Or perhaps it was the “keep your enemies closer” theory.
That particular Dem, btw, actually already had a bad track record of abuse of power under Clinton, clearly at the orders of Clinton or Hillary.
He seems to have done nothing harmful under Bush, but as soon as the Dems were back in power, he went back to his old ways. However, he wouldn’t have done it without marching orders from someone superior to him.