The leading case on the failure to preserve evidence is R. v. La,  2 S.C.R. 680, the principles of which are summarized in R v Bero (2000), 151 CCC (3d) 545, 39 CR (5th) 291 (Ont CA), at para 30
In order to establish "unacceptable negligence", it is not necessary to show that the conduct in question was sufficient to shock the conscience of the average citizen.
Rather, conduct that shocks the conscience of the average citizen is relevant to determining whether a failure to meet the disclosure obligation constitutes an abuse of process.
Note, however, that it is not necessary that an accused establish abuse of process for the Crown to have failed to meet its s. 7 obligation to disclose”: R v Laing, 2016 ONCA 184 at para 36
The police are not held to a standard of perfection in their duty to preserve evidence. Evidence - even relevant evidence - will occasionally be lost by the police whose conduct was nonetheless reasonable. But the degree of care expected of the police increases with along with the relevance of the evidence: R v Laing, 2016 ONCA 184 at para 37