My understanding is, that you can use deadly force if necessary to defend a dwelling. The people watching out for the property for the rightful owner would probably do well to come back armed and forcibly remove them, with some sort of written authorization from the owners. They can call the cops if they dare.
You can but if the DA says you can't (think a Soros DA), then you may be on trial for murder, despite what the law allows.
Gotta be careful here. If you are in the house and someone comes in illegally, you are typically allowed to use deadly force since it's presumed that you're at risk of imminent bodily harm. However, if someone is in your house when you're not also inside, use of deadly force would likely get you into trouble because at that point you're defending only property. Deadly force can't be used in the defense of property. So if you come home from vacation, notice someone in your house and go in with guns blazing, you're likely to catch a homicide charge.
That is true for the person whose dwelling it is; not for his neighbors.