I may assume that the intelligence evidence is compelling without being shown it, but I understand that it can't be released to the general public. It has been released to the British, hence the significance of the British government and not ours publishing the White Paper. I know that it is just to kill civilians in Afghanistan under the circumstances explained in the article.
Here are the preface and the conclusion of the White Paper; the entire text is here: RESPONSIBILITY FOR THE TERRORIST ATROCITIES IN THE UNITED STATES, 11 SEPTEMBER 2001 - AN UPDATED ACCOUNT.
This document does not purport to provide a prosecutable case against Usama Bin Laden in a court of law. Intelligence often cannot be used evidentially, due both to the strict rules of admissibility and to the need to protect the safety of sources. But on the basis of all the information available HMG is confident of its conclusions as expressed in this document.
RESPONSIBILITY FOR THE TERRORIST ATROCITIES IN THE UNITED STATES, 11 SEPTEMBER 2001 - AN UPDATED ACCOUNT
INTRODUCTION
1. The clear conclusions reached by the government are:
- Usama Bin Laden and Al Qaida, the terrorist network which he heads, planned and carried out the atrocities on 11 September 2001;
- Usama Bin Laden and Al Qaida retain the will and resources to carry out further atrocities;
- the United Kingdom, and United Kingdom nationals are potential targets; and
- Usama Bin Laden and Al Qaida were able to commit these atrocities because of their close alliance with the Taleban regime, which allowed them to operate with impunity in pursuing their terrorist activity.
2. The material in respect of 1998 and the USS Cole comes from indictments and intelligence sources. The material in respect of 11 September comes from intelligence and the criminal investigation to date. The details of some aspects cannot be given, but the facts are clear from the intelligence.
3. The document does not contain the totality of the material known to HMG, given the continuing and absolute need to protect intelligence sources.
[...]
Conclusion
74. The attacks of the 11 September 2001 were planned and carried out by Al Qaida, an organisation whose head is Usama Bin Laden. That organisation has the will, and the resources, to execute further attacks of similar scale. Both the United States and its close allies are targets for such attacks. The attack could not have occurred without the alliance between the Taleban and Usama Bin Laden, which allowed Bin Laden to operate freely in Afghanistan, promoting, planning and executing terrorist activity.
While not sufficient as a prosecutorial case (for same imaginary world government jurisdiction), it meets the standard of reasonableness, necessary to show just cause.
I know that it is just to kill civilians in Afghanistan under the circumstances explained in the article.
No you don't. Civilians are not responsible for what a Saudi National planned while inside their borders (if in fact he really did).