Why was qin shi huang so superstitious?

Actually, lack of the awareness of the world, many people in the ancient China were very superstitous. They believed everything they had was blessed by god. Emperors are said to be the son of the heavenly god, so there is no wonder they can be that superstitous.

QSH superstitious believed does not necessary comes from himself. If you know about the 3 kingdoms era in China, many will believes Cao Cao is the bad guy. If you look at all these in today's context to understand why certain judgement call were made, you should understand it better.

During warring state era, there was no peace. There after were short period of peace after QSH unify China but 2 critical issues happen there after.
1 - soon he discovered that managing 1 state is totally different from the size whole China. Actual reports won't reach his ear. Newly appointed selfish ministers will do whatever it takes to protect their own benefits and position (just like every other historical kingdoms)
2 - too soon and too early to further defend the newly form country by building Great Wall of China. Imagine after years of bloody fierce war and the construction of the wall follow thru short after spreading from Beijing to Gansu (kilometres and kilometres of heavy stones) on the hilltop and at that time of technology and sciences to build such thing is too much