What exactly is it that Le Pen wants to do as President of France that makes her so vilified on here? Sorry, I know nothing much about this election.
I'll take this at face value although I'm surprised anyone educated needs to ask.
Quite apart from her policies - which are basically far left economics (anti-free trade, pro nationalisation - basically Corbyn on steroids, except that her policies would explicitly favour white French people) with right-of-Ukip social policies. MLP has attempted to improve its public image, notably by sacking her father from the party, but many of its officials are tainted by a similar history of anti-semitism and racism, including holocaust denial (négationnisme in French).
If you were in the least offended by the Ken Livingstone saga, you should be aware that what he said was pretty mild compared to what many FN politicians have said in the past. The new FN president is on record as doubting that the holocaust happened, or at least happened as historians currently believe (he doubts that Jewish people were gassed with Zyklon B), and also has a history of Pétainism (support for the occupied Nazi-collaborator government in WWII).
Only last week Marine Le Pen made waves by saying that the French were not responsible for the treatment of Jews under the occupation - that there was no national responsibility for the Vel d'Hiv, in which thousands of Jewish people were rounded up by the French (not by the Nazis).
A Le Pen presidency would probably be similar to Trump, in that most of her policies will be very difficult to implement - because the money and the political support would be lacking, and because a lot of what she wants to do is objectively very complex (if the UK leaving the EU will be hard, it would be 100 times more complex for France, and it has much less public support) - so very little concrete will get done. But at the same time, as in the US, the state machine would roll back openness, individual freedoms and the rule of law by limiting press access to government (MLP is already "doing a trump" by refusing to allow some media to cover her campaign), cracking down on personal liberties particularly religious freedoms and women's and minority rights, and attempting to circumvent the courts. She is under several judicial investigations and the European Parliament has begun the process of lifting her immunity - but if she were elected she would benefit from a much wider immunity as a head of state.
Is that scary enough or do you want more?!