Pretty much just embarrassment, although it could also be to prevent having to testify. He of course knew the kinds of things he and his goons (many of whom coincidentally also work or worked for Trump) got up to and in an impeachment a lot of that would potentially become public. He was likely done either way and at least this way nobody would ever know exactly how bad it really was.

There have been three distinct turning points for the Republican party. The first was Nixon which marked the point at which Republicans stopped trying to compete on good policy and started trying to weaponize hate and fear as well as went all in on dirty tricks. The second turning point was Reagan which was when they cemented their messaging around trickle down economics (formerly known as horse and sparrow theory) and really began pushing hard for late stage capitalism. The third was the election of Trump which is when the party abandoned all pretense of decorum and went full mask off on their greed and corruption.