Bitcoin really secure?What if a hacker was able to hack Bitcoin system/blockchain?
This is impossible a hacker simply can't hack the blockchain there are lot of things involved the only risk is the hacker can ddos blockchain which can stop transactions other than that nothing can be done to the blockchain.
Who manages Bitcoin security and what are the security measures taken to avoid hackers being able to steal Bitcoins?
No one there is no owner of bitcoin , blockchain is just a software created by a person named satoshi he had created this system and that's it no management is required people can earn by mining and can send transactions with a fees the fees goes to miner and the system is designed in a way that it awards them with small rewards.
Why Bitcoin is not blocked for those who use it illegally by buying/selling in the deepweb market?
As i said no one controls bitcoin that's the reason it is anonymous it can be bad at sometimes as in the case of deep web or online scams but even if we see a scam going on nothing can be done.
What if governments ban Bitcoin because of these people?
There is no way of blocking it all they can do is shut down exchanges or block them the government can't do anything else bitcoin can never be shut down.