It's rather trite to say that everyone should assist in the pursuit of lawbreakers etc. But we also have to remember there's supposed to be - and I'd like to think still is - a concept of presumption of innocence.
The FBI appear to have chosen the case on which to raise their demand with considerable care. There is nobody charged and very likely nobody ever to be charged as a result of this. The user of the phone, whilst neither charged nor convicted, has any outstanding human rights to be contradicted, moreover it's likely that when a coroners court sits on the murders it's likely to pronounce that he committed them. Also the phone wasn't his property, it belonged to the local government body who are agreed to the phone being hacked. So, apart from the fact that the FBI and the owners between them made a cock-up by changing the password and the dubious arguments for the phone's likely evidential value over and above any information the FBI might already have, the case for doing this is about as persuasive as it gets.
However, the precedent it would set, practically if not legally, would extend well beyond these circumstances. Even if a decision in favour of the FBI were limited to the particular circumstances I outlined above it would still be a dangerous precedent. On the one hand it would undoubtedly be just the first slice in a campaign of salami tactics to make the decision universal. On the other, if the circumstances were limited to those in which the user were dead that might be an irresistible temptation that shouldn't be on offer.
The argument's been made that those who break the law shouldn't be entitled to call on the law to protect them. That argument fails to take account of the presumption of innocence. Until proven guilty the alleged lawbreaker is as entitled to the protection of the law as anyone - it's one of the final lines of defence we all share against a false allegation. So the risk of such a precedent being widened to overrule that presumption is not a trivial one.
If we are to be called on to assist against lawbreakers we need to be able to trust those who make such calls. As things stand various agencies in both the US and the UK have forfeited a great deal of public trust. ISTM that one of the most important things now, for the FBI and for the others, is to rebuild that trust. In the circumstances, whatever new evidence might be gleaned from the phone the wisest step the FBI could take right now would be away from their request. It could be the first step towards that rebuilding.
As the FBI and their supporters have chosen to invoke the rulings of Edward I we should remember that the presumption of innocence was reintroduced into European law in his time and also that he not only reaffirmed Magna Carta, he made it part of English statute law. From Magna Carta we have the concept of due process of law. These days I fear the concept of due process is being stretched to breaking point if not beyond.
Finally I should reiterate that I spend a good many years as a forensic scientist in the midst of a terrorist campaign. I carry no brief for terrorism or any other form of criminality. I understand from my own experience the desire to investigate cases as fully as possible. But the thing I dreaded for all those years was the possibility that, however inadvertently, I might end up making a mistake that could help convict someone who was, and would know themselves to be, innocent. I wish I could see evidence of that dread in the decision makers of law enforcement agencies today.