Trump evidently thought he could get away with firing FBI director James Comey, because Democrats weren't happy with him either. He was wrong though. The reason he was wrong is not because everyone thought Comey was a great FBI director, but because of the timing of the firing. It came as the investigation into Trump's Russian scandal was starting to pick up steam -- and many see Trump's action as an attempt to stop, or at least slow that investigation.
Trump pretty much admitted that himself, as he told NBC on Thursday:
"When I decided to just do it, I said to myself, I said 'You know, this Russia thing with Trump and Russia is a made-up story, it's an excuse by the Democrats for having lost an election that they should have won.' "
It's not Trump's place to say whether the investigation is wrong, or that it should be stopped. He is the one being investigated, and any attempt to involve himself in the investigation in any way approaches an impeachable offense.
Trump also tried to impede the investigation in another way also. After inviting Comey to have dinner with him, Trump demanded that Comey pledge his loyalty to him -- and he asked for that several times. When Comey refused, as he should have done, I think his job was as good as gone -- and indeed, it was just a few days later that he was fired.
This is extremely troubling. It may be acceptable in an authoritarian regime to demand loyalty to a person. Most dictators (Hitler, Mussolini, Stalin, etc) have demanded that government employees and citizens swear loyalty to them. But things don't work that way in a democracy. In a democracy, loyalty is sworn to the country and the Constitution -- not the government or any particular leader in that government.
Trump has shown once again that he really doesn't understand (or care about) our democracy. He wants to be an authoritarian leader who can do anything he wants to do -- and it frustrates him that our democracy won't allow that.