There are two things to point out here:
1) With Joe Biden running for president in 2020, President Donald Trump should not have directly involved himself in any investigation of Biden, because Trump had too much to gain professionally from the outcome of such an investigation. Such involvement by the president (who would have a personal stake in the outcome) would not accord Biden equal justice under the law. Sen. Lamar Alexander explained this very well in his statement on impeachment.
2) President Donald Trump also should not have withheld military aid to the Ukraine, previously authorized by Congress, in order to encourage such an investigation. President Trump simply should not have involved the messiness of American politics in the serious business of foreign affairs - but he did. Once again, Sen. Lamar Alexander covered this in his statement.
There is nothing wrong with investigating Joe Biden and/or his son, Hunter. However, President Trump should not have directly involved himself in any such investigation. If the conduct of Joe Biden and/or his son warranted an investigation, it should have been left up to the Attorney General, William Barr and the Director of the FBI, Christopher Wray. President Trump should have stayed out of it... but it is clear in the transcript of the July 25th phone call with President Zelensky of the Ukraine, that he didn't.