"Don't get too comfortable with laws and checks and balances you believe are bullet proof."
Nothing is "bullet proof". It requires constant vigilance, action and, above all, a sense of duty and responsibility. Your example of violence as a subversive force in society is simplistic. You see, there is a vast chasm between what you have described -- an independent criminal, or even a gang of them -- and institutional oppression. There is a reason why western societies have been able to survive individual acts of violence and that reason is a faith in their democratic institutions and laws (and, by extension, in those who enforce the laws). Criminal violence is very different from situations where the authorities are the ones committing the violence and oppression. We can easily shrug aside deranged individual violence and we do our best to prevent/fix criminal violence. But a society cannot survive when the police and other institutions of government, themselves, are the instigators of violence against citizens.
The legal system cannot be destroyed by individual violence because the perpetrators of that violence would be hunted and effectively removed. Judges in free societies do not fear for their lives for the very reason that there is confidence in the enforcers of law and order. The legal system and justice can only be eroded and destroyed from within. Sadly, with Americans ever more eager to embrace "strong man" tactics and an outlook of "might is right", the day these institutions are rendered ineffective may be closer than we would like. The polarization of American society threatens to tear it apart. I understand differences of opinion, but the outright hatred and the spread of it in the echo chambers of the internet, is disturbing. We don't have to think alike, but we should protect each other's rights to think and speak freely (in a manner that is legal and without hate). When we gloat at the violence inflicted on those we don't like, and worse yet, when we promote violence against others, we destroy the very thing that keeps us free. It is not done by some random criminal with a gun, though that is a favourite trope in scaremongering.
To bring this back to Trump, a judicial application of considered checks and balances, supported by the populace, would keep him in check effectively. And there is still no chance in hell he will be accepted and elected even by the majority of the Republican voter base, much less the entire electorate. Once the primaries begin it is my prediction that, unless he finds a source of serious funding, he will be out of the running after the first 5-6 contests. At that point, only a stubborn application of money, or money combined with a renewed bid as an independent candidate, will keep Trump around.