It is rare for so many high-profile Republicans to repudiate their own party's candidate, in many cases at significant personal risk to themselves. These are people who have connections, and who have been paying attention far more than you or I ever could; what do they know that you're not paying attention to?
There's a more complete list with citations on Wikipedia; these are just some of the highlights.
“The American people deserve to know that President Trump asked me to put him over my oath to the Constitution. [...] I think anyone who puts themselves over the Constitution should never be president of the United States.” — 6 August 2023
“In each of these cases Donald Trump is pursuing and articulating an agenda that is at odds with the conservative agenda that we governed on during our four years. [...] I cannot in good conscience endorse Donald Trump in this campaign.” — 15 March 2024
“He is the most dangerous person ever. I had suspicions when I talked to you about his mental decline and so forth, but now I realize he’s a total fascist. He is the most dangerous person to this country.”
In our nation’s 248-year history, there has never been an individual who is a greater threat to our republic than Donald Trump. He tried to steal the last election using lies and violence to keep himself in power after the voters had rejected him. He can never be trusted with power again. As citizens, we each have a duty to put country above partisanship to defend our Constitution. That is why I will be casting my vote for Vice President Kamala Harris.
“Rejecting the results of an election is as un-American as it gets. To someone like me who talks to people all over the world and still knows America is the shining city on a hill, calling America is a trash can for the world is so unpatriotic, it makes me furious. And I will always be an American before I am a Republican. That’s why, this week, I am voting for Kamala Harris and Tim Walz.”
Not convinced by this lineup? Here are other Reasons You Might Want to Vote for Trump and Why You Should Vote Kamala Instead