Speaking on 24-hour news channel i-Télé on Wednesday morning, Le Pen said she would hold a referendum on the subject if elected.
She repeated her calls on Thursday morning in an interview with radio station France Info.
Saying she would prefer to see such criminals "leaving prison feet first only", she called for "either the death penalty or real life imprisonment."
"I ask myself whether, if this man is arrested, we might end up seeing him crossing the road in 27 years."
Le Pen has also reacted angrily to what she believes is "negligence" in the case of Mohamed Merah.
Merah is the chief suspect in the seven shootings in Montauban and Toulouse and was stilled holed up in an apartment in Toulouse on Thursday morning.
"At first glance, it seems that there are problems of negligence in this affair," she said.
"This man has been convicted 15 times, we know he has been to Afghanistan and that he has threatened people and shown decapitation videos," she added.
"We're told there are fifteen people with his profile, but shouldn't he be watched a bit more attentively?"
Le Pen had initially been silent when news broke of the killings, with some politicians suggesting the killer was likely to be a far right sympathiser.
Since the identity of the chief suspect has been made public, she has been robustly defending her policies on French television and radio.
Le Pen also attended the memorial in honour of the three soldiers who were shot dead on March 11th and 15th.