But I Think the criminals like only one thing in bitcoin which is its feature of anonymity , with that they can buy any thing like weapons and drugs etc and will remain anonym to the sellers etc.

Yes, they will make benefit of any currency they find and like for their business, and only governments can control them by giving opportunities for their citizens so that they are not trapped by criminals to make more criminals.

I don't know how to make it possible to determine  illegal transaction. Also i don't know who cares about it? Goverment? meybe. But in some countires even bitcoin is illegal, so hard to make it possible 

I think it's not easy to governments and developers to make Bitcoin such way, because simply it's decentralized currency which no one can control it or know the identity of the sender and receiver

The people who used fiat to ilegal activities will keep doing that ,only those who never made ilegal things may try it with bitcoin,they has their system to make the house clean paying politcians ,police anyone who can make a hit to their business.Bitcoin may be used but in the most is online ilegal,casinos,exchanges,porn.

Yes, we do not have to blame any currency, they are not created for the criminal to do their crimes but for the convenience of people, but are taken by criminals and started to benefit from it.

It can't be controled. Knife is not created to do cirmes but criminals using it. Bitcoin is just a tool. Developers can't determine who did illegal transaction.

Sure. Also heroin. Heroin is just a tool which wasn't created to make junkies. But make junkies it did.
That's why it's B& nao.

Bitcoin is probably being used for illegal activities in the world today except it would be very hard for anyone to find out and in the future these people will be able to think up more ingenious ways of doing it.