He did a huge amount of harm to our government. Not quite like this time, where most of what he is doing is outright illegal and is essentially a soft coup, but really bad nonetheless - just mostly aimed at making him money and getting/keeping political power instead of destryoying the country. Much of that was outright illegal, but a lot of it was just breaches of "norms" and "decorum".
I literally can't fit it all into one comment, its so much and such a convoluted web of schemes and lies and crimes and support from other politicians/lawyers/the media. And every day was something new. I followed all the legal cases relating to his admin back in the first term - it was hard to keep up with even while it was all happening. Much of the reason he was never charged or indicted for so much of what he did is that you can't criminally indict a sitting president.
The Mueller investigation into the Trump administration's conduct with Russian political operatives found that he more than likely illegally colluded with Russia to the detriment of the US and to defraud and disenfranchise voters, but literally couldn't charge Trump since he was a sitting president - hoping instead that someone would pick up the investigation when he could be charged. It is notable that that investigation produced 37 indictments and 7 convictions/guilty pleas, referred 14 more cases to DoJ for prosecution, and recovered like $48M in misappropriated government funds (the investigation cost $32M, so it was actually profitable). So, this investigation couldn't prosecute Trump, but 34 people in his administration were indicted and the findings of the report suggested they would have prosecuted Trump if they were legally allowed to. That says all you need to know, IMO.