A politician’s first loyalty should be to the country, not to some other politician in a “you scratch my back and I’ll scratch yours” mindset. I have zero problem with a former underling running against a former president. And that’s regardless of who the president is.
Loyalty is important to Trump. Loyalty is not important to politicians.
In government, loyalty to the country and your ideals is more important than loyalty to an individual. We shouldn’t elevate personal loyalty to a political leader as some kind of ultimate virtue, because it isn’t.