There is no such thing as time travel. You can see the past, you
can't change the past.
Anyway here's the full story of Mogallana attempting to save the
500 people:
http://honsing.com/Disciples.htm
Mogallana (Also Moggallana, Maha-Maudgalyayana, foremost in
possessing supernatural powers)
Mogallana followed the Buddha together with Sariputra. Mogallana
gained Enlightenment within 7 days and attained supernatural
powers. Why did Mogallana have such great supernatural powers? This
is because in a previous live, Mogallana gave a treat to a Pacceka
Buddha. The Pika Buddha was not very eloquent in explaining the
Dharma through words, hence he illustrated using supernatural
powers. Mogallana was impressed and vowed that he too would achieve
in supernatural powers to propagate the Dharma.
Supernatural powers were quite common among Buddha's disciples.
However the display of it was always discouraged by the Buddha, all
except for Mogallana, he was even praised for using supernatural
powers in propagating the Dharma. There were many interesting
stories, including one giving rise to the Buddhist festival of
Ullambana. However I shall mention one that is less heard of.
There was once where Kapilavastu was to be attacked by King
Virudhara from the Kingdom of Kosala King. Buddha tried to prevent
the war by sitting at the pathway leading to Kapilavastu on 3
separate occasions. King Virudhara always retreated whenever he saw
Buddha. However King Virudhara's revenge and hatred could not be
subdued. Buddha knew that there was no way to prevent the Karma
further, thus he left them alone.
Mogallana had been successful in all his supernatural powered
deeds. Hence he thought that more could be done in this case.
Buddha explained to him that this was due to the past bad Karma of
the people in Kapilavastu. The people remained arrogant,
non-rependent, deceitful, and did not amend their ways, hence there
was no way of substituting the fruits of Karma. Mogallana was
unconvinced. The army surrounded Kapilavastu such that no one could
escape. Mogallana used his supernatural powers to fly into the
city, found 500 great men, transform them into his alms bowl, and
flew out of the city to a safe spot. However when he opened the
bowl to recover the men, he found a bow of blood water instead!
Mogallana realized that supernatural powers cannot overcome Karma.
(For more details, check out here for more details:
http://www.geocities.com/Tokyo/Courtyard/1652/Massacre.html.)
This was a lesson for Mogallana. Finally one day Mogallana was
ambushed by some Jain followers. They pushed rocks from a hill onto
Mogallana. Mogallana knew this was his Karma and he did not use
supernatural powers to overcome it. He died under the rocks. This
was before Buddha's final Nirvana.