It's clearly fair to discuss the institutional failings of the Met that allowed him to masquerade as being on duty in order to carry out this particular crime in this particular way. If the culture of the Met was different and warning signs were heeded, this crime would not have occured in the way it did.
Why is it therefore not fair to discuss the law that he exploited in order to select his victim? The fact that the law was temporary, criminalised normally legal, innocuous actions, often left people feeling confused about what the actual law was and had frequently been over-zealously enforced across many police forces is very relevant to the exact way that this crime occurred.
Criticising bad law does not change the fact that the culpribility falls entirely on the offender. It might mean that better legal choices are made in the future.
If this law had not existed he probably would have selected a different victim with a different ruse, but it probably would have been logistically more awkward and risky for him as the legal position would likely have been clearer and warning signs may have easier to read in a more conventional situation.
There is also an irony in the way lockdown laws were used to heavily handedly police the vigil afterwards, and the excessive policing of many of the women who came to pay their respects. Not good PR for an organisation now facing accusations of institutional misogeny.
I've never made any secret of being critical of lockdown and the range of negative consequences that it caused (and continues to do so in the aftermath of heavily restricted public services). Ultimately many examples of poor policing have undermined public confidence in the police service through the past 18m and that does merit wider discussion.
If we can't criticise bad law and its role in facilitating many tragic incidents and crimes, we are condemning ourselves to blindly repeating these errors and wasting more lives in the future.