i dont know the particulars...they could always come back with a gun, now they know where you live etc...if someone threatened my family i am not sure i would react very differently. i guess the dead men will not speak and we only have the accused's version of things. if i knew for certain that a person was only going to rob then of course i would not kill someone. but i dont see how you can know that or trust the work of a person who has already broken in and is in the process of stealing.
Of course they would not come back with a gun, unless they are loaded on some fucked up drug or have some kind of grudge against you.
A robber usually tries to get what they can without confronting anybody. Usually. They are not there to terrorize. They are there to rob, and get the fuck out.
But yes, usually they arm themselves.