Representatives also take an oath to support the constitution.
The man tried to stay in office after losing an election. It's literally about the most dangerous thing a president could do for the future of this democracy. Democracies are fragile. We're the longest running one, and we're not that old. Our democracy will surely fail one day, and it will quite likely look something like what Trump tried to do, only done more effectively.
No doubt Cheney believes she is acting in the best interest of her constituents by protecting the structure of the country as a whole. She's not acting in her own best interest to stay in power. Not everything is a personal vendetta, Mr Monkey. It's a valid take to view Trump as a serious threat to our whole system.