Imagine a bank robber killing 5 people and holding 20 people hostage. As soon as he takes one step outside the police will shoot him on sight, WITHOUT any trial. It's the same thing basically.
They're targeting these guys for assasination for things they might do, or are suspected of doing in the past. It's not like they catch them in the actual act of committing terrorism.
So to satisfy your Strawman it would be the equivalent of finding a bank robber months after the robbery inside his house, and the police blowing it up with his family inside to get him. I don't think that would fly in any reasonable society
Edit: Interestingly here is a RL example that happened today, terrorists caught robbing banks, beaten by the cops then mugshots digitally altered so nobody would see the wounds http://www.guardian.co.uk/world/2013/feb/04/greece-police-lo...
- Poses an immediate threat to the public or the officer
- there is no weapon in sight, or
- the weapon is pointing at the ground etc.
Obviously preserving human life trumps any other concerns, this includes the "bad guy's" life.
If, however, the bank robber steps out unarmed and is shot and killed, the family should have the right to bring a wrongful death suit (or something, IANAL) against the police officers in question.