Shia said mahdi did not hide due to fear, but

[Hadrami]

apart on what is clearly mentioned in their hadith that he is afraid to be killed, the respected leader of their sect

al-Tusi in his book al-Ghaybah page 203, al-Murtada in his book al-Shafi volume 4 page 149 and al-Mufid in his book al-Fusoul al-Mukhtarah page 395:

لا سبب للغيبة ولا علة تمنعه من الظهور الا خوف الامام على نفسه من القتل
 
“The only reason behind the ghaybah (occultation) of the Mahdi and the reason that he cannot appear is that he fears that he will be killed.”

http://www.twelvershia.net/2015/09/03/who-was-the-bravest-companion-ibn-hazm/

Will the shia say he was slandering the mahdi too?

[AbdulKhader]

The concept of Ghaibat-al-Sughra seems to be the idea that the fictional Twelfth Imam went underground to escape Abbasid persecution. It is not a metaphysical doctrine, basically similar to Osama bin Laden or Mulla Omar going underground to escape the Americans, by hiding in an undisclosed location somewhere in the mountains etc., but sending out messages through their agents. The only difference is the so-called agents of the Twelfth Imam were the ones who concocted his existence in the first place. There was no 'Twelfth Imam' because the Eleventh Imam Hasan al-Zaki^RA never had any children and he never even had a wife (he died in his mid-20s too).

During the Ghaibat-al-Sughra, the fictional Twelfth Imam had four main agents, and they in turn had their own representatives, all moving about clandestinely with messages for the broader Shiite community. Of course these messages were always about financial matters relating to collection of Khums, Zakat, etc., and there was never any kind of knowledge about Quran and Sunna or any other religious wisdom conveyed in these "Tawqee'aat" of the fictional Twelfth Imam. Apart from the business of collecting money these messages from the fictional Imam also dealt with rebuking false agents and envoys claiming to be in contact with the Twelfth Imam (to secure the coffers of the real agents of the fictional Imam and destroy the rivals who were trying to play their game too!)

Now after some 70 years passed and the last of these four agents died, people began to wander, the Twelfth Imam must be a very old man by now and nearing his natural death, and he still hasn't appeared? What's going on?

Then the idea of a Ghaibat-al-Kubra was invented to solve this dilemma, and now its been some 11 centuries and still the fictional Imam hasn't appeared (because even if he did exist he'd be long dead by now). But if its admitted that the fictional Imam either didn't exist in the first place, or if he did exist he died and therefore couldn't have been the true "Qa'im" and "Mahdi", then the entire religion of Twelver Shiism collapses, so we have to invent another doctrine called "Ghaibat-al-Kubra" and now the disappearance of the fictional Imam is mythologized and the Imam becomes like an omnipresent spirit with supernatural powers to stay alive and invisible for centuries!

[Hadrami]

major occultation was a great con act, but very beneficial for shia, because it saves them from fighting each other again on who is the next imam every time an imam died