Actually it's the other way round, the suspect is innocent and it's up to the police and CPS to prove he is guilty.
Treating a death as murder is presumptuous and prejudicial to the outcome of a proper investigation. Unless there's some aspect of the home owner's story that is suspicious e.g. saying the deceased was breaking in when there's no evidence of a break in, why should the police assume the home owner is lying?
That's why it's called suspicion of murder. And it's a reasonable suspicion when a person has been stabbed to death, as in general terms, and under our legal system, stabbing someone to death is a disproportionate response to their trying to rob you.