This sort of thread seems to pop up every time someone kills a burglar with the same indignation and cries of "why arrests the home owner" and "serves the burglar right".
As has been said above the Police are doing their job, they've got a dead body after violence, they have no idea what happened just potentially a story from someone, so they have to preserve any evidence in case it's needed including statements from the person that used force, and (this is key) also make sure they preserve the rights of the person who used force.
Arresting the homeowner does both, and I imagine when it looks like a fairly obvious case of self defence the officers will likely be very clear that it's mainly a formality and probably try to comfort the person if needed (most people probably won't be over joyed at the realisation they've killed someone).