For me, it's not about breaking the law that's the fundamental problem; as others have also said it's because he then lies about it. I've had points on my license for speeding, but I didn't lie about it when I was caught because I know that would only make things worse. But Boris's life experience seems to have taught him that it's worth lying to avoid punishment, and he's already been sacked twice for this. It is fundamentally what makes him unfit to be my prime minister, and I have always thought so, and I'd like to think that even if I wanted to leave the EU I'd have thought so, but too many people were prepared to overlook his rampant fabrications in the eye of the "prize" of leaving the EU which his predecessor was unable to accomplish.
If he'd come clean at the start we'd have moved on by now. The crime wasn't major, although it was hypocritical to be found out doing things the rest of us didn't do, and I think at the time his instinct was to lie to avoid any charges of this. This decision has mainly unravelled although he still seems to be "getting away with it"; I feel that his actions have blighted the chances of his party for the foreseeable future but I could be wrong since I'm biased to loathe the man and hold him in contempt.