Bitcoin's anonymity is really a great characteristic. It enables users to use BTC in any transactions with nearly no traces. But is this really a good thing? Let us consider utilizing bitcoin in a negative way. Since bitcoin is considered currency, there is no doubt that a person that needs money will not reject bitcoin even if it involves unethical or illegal act.
Due to the anonymity of bitcoin, some uses bitcoin in a negative way [citation needed]. They can easily use bitcoin in paying others to commit crimes. They can even use bitcoin in bribery and/or money laundering. This characteristic of bitcoin are abused [citation needed] and it will never end well.
For the case of terrorism, terrorist can easily purchase weaponry from other country. With the use of transaction using bitcoin, authorities will have a hard time tracing that transaction thus, will give advantage to the side of terrorists.
This is not an impossible scenario. People can now use bitcoin in any way they want to. It's inevitable.
It is the sad truth.
As long as bitcoin exists, there will always be people using it for illegal purposes.
Much like as long as cash exists, people will probably be using it the wrong way as well.
Google started out great because people would be able to search for pornographic videos and photos on the internet which was basically all there is on the internet at that stage easier. Then it developed into something way more than that, an advertising platform complete with a cloud drive, customised results for each user based on their profile, etc.
So I would personally expect bitcoin to be something similar to how google started out - used for questionable purposes, sure. But in the end the good will outweigh the bad pretty dramatically.