Only an utter fool thinks there's meaningful parity between these: that comparison is a peanut gallery stunt, not a serious argument.
Hillary only called Trump "illegitimate", and in the wider context of gerrymandering, voter suppression, etc. She didn't orchestrate a series of schemes and scams including encouraging a mob to storm the Capitol in order to subvert the election.
I think you are confused about how the law works. We can't know whether something is legal or not until it is tested in court.
At face value, there is law which can be interpreted to state that someone who has carried out an insurrection against the USA may not stand as a presidential candidate. There are clearly reasonable grounds to accuse Trump of this, and it is right and proper the courts test what the limits of this law are, which will then establish precedent. So cases have been brought, and at least one court found it credible enough. It will then go to a higher level court as it should. It may well be rejected, but it is asinine to think no-one should, or would, ask such an important question under the circumstances.
Also key to your misrepresentation of this is that Democratic states have not been initiating these attempts to remove Trump from the ballot. They are being initiated by voters, in many cases (e.g. Colorado) Republican ones.
It's the law, and the USA is supposedly founded on the basis of the rule of law.
Our democracies also don't permit the 51% who won an election to murder the 49% who lost, but complaining that isn't "democratic" is neither intelligent nor interesting.