People have every right to vote how they wish and should not face abuse or recrimination for the box they choose to cross, I, and plenty of others on this forum would condemn and pull up behaviour of that kind.
2019 was a strange election in that a man with who was genuinely seen as a security risk by MI6 due to his several unsolicited off the record meetings with foreign persons of interest, a man who had lost several jobs because of his dishonesty and his inability to tell the truth or take responsibility for his actions and an individual whose private life and financial affairs were in such a state that he has the largest dossier of any politician ever within MI5 and was undergoing HRMC investigation was deemed the best option to govern the UK, for me regardless of policy, his personal unsuitability for the role would have made it impossible for me to vote for.
That many unproven or exaggerated smears against Corbyn were believed, whilst proven and factual stories about Johnson were seemingly ignored by people is something that makes little sense, I can't personally square the circle that if Corbyn was unsuitable to govern then Johnson was, perhaps both were unsuitable, perhaps both were ok, but if you had to chose one surely the one without the security compromises was the better choice.
My personal view is that the Boris brand was helped massively by a predominantly right wing written press, his presence on the winning side of the Brexit vote was seen as a tipping point to success and with careful media management cast Johnson as someone with the Midas Touch of electability, he was pardoned grilling interviews and refused to appear at certain hustings, in his own mind he became bigger than the Party itself, I think some were convinced that given the role he had craved for so long would lead to a change in behaviour and that his maverick streak was actual brilliance rather than an inability to do detail and nail to the floor things of importance. The politically errant man settling down with the birth of his Premiership.
Johnson let down all those who put a cross against a Conservative candidates name, he acted in his own interests over the those of his country, his Party and his people, a consummate conman who has spent his whole political life in deception and plot, I think the main factor was that Johnson lied his way to power and many good people trusted his lies and excused him until the penny dropped that he was never going to change and that integrity was a concept that had bypassed him and wasn't ever going to be a part of his makeup.