A selection of key paragraphs can be found below the judgment.
116. For the Court, and bearing in mind the difficulties involved in policing modern societies, the unpredictability of human conduct and the operational choices which must be made in terms of priorities and resources, such an obligation must be interpreted in a way which does not impose an impossible or disproportionate burden on the authorities. Accordingly, not every claimed risk to life can entail for the authorities a Convention requirement to take operational measures to prevent that risk from materialising. Another relevant consideration is the need to ensure that the police exercise their powers to control and prevent crime in a manner which fully respects the due process and other guarantees which legitimately place restraints on the scope of their action to investigate crime and bring offenders to justice, including the guarantees contained in Articles 5 and 8 of the Convention.
In the opinion of the Court where there is an allegation that the authorities have violated their positive obligation to protect the right to life in the context of their above-mentioned duty to prevent and suppress offences against the person (see paragraph 115 above), it must be established to its satisfaction that the authorities knew or ought to have known at the time of the existence of a real and immediate risk to the life of an identified individual or individuals from the criminal acts of a third party and that they failed to take measures within the scope of their powers which, judged reasonably, might have been expected to avoid that risk. The Court does not accept the Government’s view that the failure to perceive the risk to life in the circumstances known at the time or to take preventive measures to avoid that risk must be tantamount to gross negligence or wilful disregard of the duty to protect life (see paragraph 107 above). Such a rigid standard must be considered to be incompatible with the requirements of Article 1 of the Convention and the obligations of Contracting States under that Article to secure the practical and effective protection of the rights and freedoms laid down therein, including Article 2 (see, mutatis mutandis, the above-mentioned McCann and Others judgment, p. 45, § 146). For the Court, and having regard to the nature of the right protected by Article 2, a right fundamental in the scheme of the Convention, it is sufficient for an applicant to show that the authorities did not do all that could be reasonably expected of them to avoid a real and immediate risk to life of which they have or ought to have knowledge. This is a question which can only be answered in the light of all the circumstances of any particular case.
121. In the view of the Court the applicants have failed to point to any decisive stage in the sequence of the events leading up to the tragic shooting when it could be said that the police knew or ought to have known that the lives of the Osman family were at real and immediate risk from Paget-Lewis. While the applicants have pointed to a series of missed opportunities which would have enabled the police to neutralise the threat posed by Paget-Lewis, for example by searching his home for evidence to link him with the graffiti incident or by having him detained under the Mental Health Act 1983 or by taking more active investigative steps following his disappearance, it cannot be said that these measures, judged reasonably, would in fact have produced that result or that a domestic court would have convicted him or ordered his detention in a psychiatric hospital on the basis of the evidence adduced before it. As noted earlier (see paragraph 116 above), the police must discharge their duties in a manner which is compatible with the rights and freedoms of individuals. In the circumstances of the present case, they cannot be criticised for attaching weight to the presumption of innocence or failing to use powers of arrest, search and seizure having regard to their reasonably held view that they lacked at relevant times the required standard of suspicion to use those powers or that any action taken would in fact have produced concrete results.
122. For the above reasons, the Court concludes that there has been no violation of Article 2 of the Convention in this case.