By all accounts the burglar was a scumbag who did time for his specialism in life which appears to be extracting valuables from old people in their own home. I haven't bothered reading the whole thread but arrests need to be justified by PACE code G
https://www.justice-ni.gov.uk/sites/default/files/publications/doj/pace-code-g-2015-.pdf
What possible grounds would the arresting constable have for a "reasonable, objective grounds for suspicion" of murder in these circumstances? The home owner should have been offered a cup of tea, protection for the night and advice on having full alarms/ alert to the police station in case of repercussions.
The Martin case is entirely different, from what I remember the guy had been lying in wait with a shotgun behind a door.
If someone comes through your door armed when you are asleep you have every right to use reasonable force to defend yourself. English law is in some ways stronger than all the "stand your ground" nonsense prevalent in some US states such as Florida. The problem appears to be inept Police failing to have common sense in implementing it. I remember a case of a sub-post master who killed a man who came into his Post Office with a shot gun using a string cutter and was not even arrested - seems entirely reasonable to me.
Postmaster's distress over killing of armed robber