Disclaimer: I don't support Le Pen. I also don't support banning her from running, but it's harder for me to understand and argue why I feel that way. I think it's a combination of:
- If people would elect a "bad" leader, disqualifying them delays the problem. They'll eventually elect another bad leader or revolt. Unless the bad candidate is only popular for a temporary reason, but that may not always be the case.
- Disqualifying someone because they broke a law is bad, because anyone can be "breaking a law"; every nation has many laws and they can be misinterpreted. Related: the situation in Turkey.
- Counterpoint: a self-interested party will disqualify whoever they want. But written laws (even guidelines) matter in the long-term because borderline party-supporters need justification to stay supportive, and people revert to laws. See: countries (like Turkey and Russia) using laws to justify banning candidates instead of "because we said so".
Keeping the rules simple (e.g. "anyone can run for office, whoever gets the most votes wins") makes it harder for an adversary to break them while retaining support. Keeping Democracy makes it more likely that an "adversary" will lose power, because such parties tend to become unpopular. If Democracy leads to a "bad" party consistently winning, why have it?
In fact, maybe it's necessary for a Democracy to have a "really bad" candidate win every once in a while, so people know what is bad. Then, the approach people should take is to ensure that the leader can't make rapid, far-reaching changes, so they can't ruin Democracy or people's lives in a single term. Just far-reaching enough for people to realize they made a mistake; then regular elections should be frequent, or there should be a quick way to get a snap election, so the bad leader is replaced.