eh? Again, you were talking about him being arrested because someone died:
it doesn't appear to have happened that way.. it seems (at least according to earlier reports of him being arrested for GBH) he was arrested and then the person died... i.e. the fact the person died isn't the initial reason for his arrest, the stabbing is... it is a straightforward point, I don't know why it has taken several posts for you to still not get it.
(again, as a disclaimer, this is on the assumption that the person acted reasonably)
In terms of what else could they have done - perhaps not drag a nearly 80 year old man off in the early hours of the morning to lock him in a cell and/or question him after he's just had his life threatened in his own home, scuffled with a burglar etc.. a rather traumatic event.
AFAIK they can question someone under caution without arresting them.
So, if there is something else that has yet to be reported, as a reason for his immediate arrest then that is one thing, but otherwise (if it is as some posters have mentioned earlier, just procedure, just doing their job etc..etc.. ) then it does seem like a rather ****ty way to treat a victim of a crime.