Regardless of any potential criminal investigations Trump is a pathological liar. How can you support that in a President?
In the letter he sent to Comey he explicitly said he had fired him under recommendation from Rosenstein. In his interview with Lester Holt he directly contradicted this.
He said a number of times his inauguration crowds were the biggest ever. This has been proven over and over not to be true.
These are just two examples of lies.
Take away anything else - the unprofesssional tweeting, discrediting the media, historical flip flopping of political allegiance, fraud etc etc and he is still a liar.
And if he lies so blatantly how can you ever trust this man with anything let alone the enormity of the American Presidency? You really want that man as your representative?