This is internet where anything is hackable.

Can we prevent it happen ?
Yes, the site for example MEW already tell you for security purpose, you should active 2FA, bookmark their website, etc

The biggest crime didn't happen yet. It will be once we let the central banks controlling the Bitcoin network. But if not, I think that it is MtGox that holds the record in the media. In term of funds for sure, all these shit ICOs are the biggest. Still, it's minimal compared to other currencies and like the currencies, Bitcoin can't be blamed but the platform people use and so these people themselves.

Just a few hacks I collected quickly from the web, as you see, only exchanges here...

Mt. Gox 850,000BTC
Bitfinex 120,000BTC
Bitcoinica 101,000BTC
Allinvain 25,000BTC
Bitfloor 24,000BTC
Bitstamp 19,000BTC
Cryptsy 313,000 LTC
Poloniex 97BTC

https://bitcointalk.org/index.php?topic=16457.0
https://en.wikipedia.org/wiki/Bitfinex_hack
https://www.hackread.com/cryptsy-hacked-bitcoin-worth-usd-6-million-stolen/
https://www.cnet.com/news/bitcoin-exchange-bitfloor-shuttered-after-virtual-heist/
https://en.wikipedia.org/wiki/Mt._Gox
https://en.bitcoinwiki.org/wiki/Value_overflow_incident
https://en.wikipedia.org/wiki/Bitfinex_hack
https://en.wikipedia.org/wiki/Bitstamp
https://www.coindesk.com/bitcoin-bitfinex-hack-multi-sig
https://www.coinbureau.com/analysis/biggest-bitcoin-hacks/

the criminals use crypto as their media to make a crime because there are many people that dont know about bitcoin 100% and they use this situation to makes a crime, if oneday bitcoin can be the popular currency i think the criminals on bitcoin will drop and drop to the bottom

I think as long as there are people who are evil and tries to scam and make easy money, they will be innovating all the way. It's just like a battle between Good and Evil, and there would never be a balance, at least for a long time. It's just like how hackers/programmers find a way to pirate an application which benefits some of the people who can't afford. I think the main difference between that is like a white hat thing vs a black hat thing.

Actually in myetherwallet scam, I am one of the stupid bitcoiner that being scam. They sent an email as the same as the email of campaign manager at bounty asking for my info like KYC and asking my private keys.I thought it was legit because campaign manager was always sent an email update to me. So next day I found out that my coin  was stole. So I can say that scam is the biggest crime here in cryptocurrency.

The biggest crime in Crypto currencies are the fact that private centralized institutions hijacked the whole experiment and then caused all these problems. Satoshi did not want people to do P2P transactions via 3rd part centralized services, he wanted people to trade 1 on 1.

These centralized services used exploitable code and hackers and thieves quickly realized this and they targeted these vulnerabilities. So Bitcoin was not responsible for these hacks, but rather 3rd party services with weak software.  

I can think of a lot of things but the infamous Mt. Gox hack was, for me, the biggest crime made in the cryptoworld. Not only does it expose a lot of flaws and vulnerabilities of the then young cryptocurrency but it also projected the dangers of having to entrust one's funds into a centralized exchange/entity. The next in line would have to be Silk Road, which was operated supposedly by Ross Ulbricht, which, again, informed the world about the possible use of bitcoin in the criminal underworld due to its pseudo-anonymity, hard to trace transactions and its overall qualities which is pretty much helpful in remaining anonymous while transacting with other people.