Ok let’s say he was casing the house (I for the record am not convinced it is him and also we don’t even know why if it is him he’s in the house) and planned to steal from this empty house later or something.

That still gives the father and son murder duo justifiable cause to shoot him or to try to do a private citizen’s arrest.

First we need to verify that that even is him and why he was at the house but above all we have to keep on the fact that it doesn’t matter if all that really happened because they still are in no way justified in murdering him.

Don’t fall into the trap of justifying horrible things with random stuff found before it happened.