Everyone in a civilised society should have the right to a fair trial if they are accused of crimes, no matter what the nature of those crimes may be. It may the case that incredibly dangerous individuals might be killed by the police in a deadly force situation and this can sometimes be appropriate. However, the police should make all efforts to bring these people in to face justice rather than summarily executing criminals without due process.
It is far too easy to make mistakes and prosecute or kill innocent people if a fair trial is not provided. If someone is an obvious, immediate threat to someone else, they should be taken out by any means necessary including lethal force. However, outside of this scenario, even a seemingly obvious terrorist deserves to stand trial and have the evidence weighed by an impartial jury. Without checks and balances like this in place, it is far too easy for an individual to be framed for a crime and taken out before all of the evidence has time to come to light.
A terror suspect caught in the act of attacking innocent people does not have a right to a trial. In other words, law enforcement has a right to protect the public from immediate threats--sometimes this means shooting an attacker to stop further harm. Now if a suspect is arrested on suspicion of being a terrorist, then he has a right to a trial.
Terror suspects do not give the individuals that they victimize a trial. They just kill them in the street without mercy. A terrorist attack is an act of war. It is not a crime. It is only appropriate that the governments involved treat terror suspects the same way that they treat their many victims.