[Keroko]

Originally Posted by Flinch

Gilles de Rais was a hero in the battle of Orleans...
Until he went crazy, he was an incredibly heroic knight who had even managed to earn the honor to wear the Fleur de Lis upon his shield.

Spoiler:

Additionally, Nero did nothing but be beyond insane.

My point is that you don't have to be a 'hero' to be summoned.

Spoiler:

As for Nero, history has a tendency to speak multiple tales. Some tales speak of Nero being well loved among the common folk, and working hard to increase Rome's diplomatic relations, culture and trade. In combat, he won the Roman/Parthian war and succesfully negotiated peace, and suppressed the British revolt.

Though come to think of it... the lack of a truly defined heroic moment might actually be Nero's motivation in the first place. Every servant has a wish after all, and perhaps Nero's wish is to die in a blaze of glory.