Killing "Terrorists"

I am watching video of the US strikes on Iraq and thinking.

In many parts of the world, "civilised" nations are fighting who they call "terrorists". Examples include Syria, Iraq, Afghan, Somalia, Yemen, Pakistan...

Let's take an example:
Suppose a UK citizen was suspected of terrorism. They must be arrested, taken to trial and proven guilty before action is taken against them. If they are found guilty then they will go to prison and eventually, if they are not longer a threat, set free.
If this is the process is what we consider to be just for a UK citizen, why is the standard applied to people in other countries so low?
An Iraqi citizen suspected of terrorism gets a missile though his window without a trial or a chance to argue. Who sees the evidence of this man's guilt?

I am not a pacifist. I know we are fighting mostly very bad people here and in those cases I am not sad that they are dead. They don't give their victims such rights, I know this, but then we are supposed to be better than them.

How do we know we are executing the right people?
What sort of evidence can we have from these wild places?
If we don't think the death penalty is appropriate for a UK citizen convicted of terrorism, how can it be right to execute someone only suspected of terrorism in other countries?
If the Pakistani government suspected a UK citizen of terrorism, would they be able to launch a missile at his house in Cambridge?
Do we say the civilian standards of justice do not apply because these people are suspected terrorists?
Can't anyone can be suspected as a terrorist, especially if the evidence for that suspicion is never made public?
There are an estimated 30,000 estimated IS members now. Should we kill them all? How many of this number have really done something terrible? Is just being with them enough?

I am not talking about collateral damage here. I mean deliberate and planned killings. I also don't mean the killing of obvious "combatants" during battles.

I mean the killing of suspects. An example might be:
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I don't mean just the UK here. Many countries have the a similar policy.

I don't have an agenda here. I am really not sure what is right or wrong in these circumstances.