Well, I do not know the law in Maryland, but if it is similar to the law in Colorado (often referred to as the "make my day law") then he has the legal right to kill an intruder if he has given warning to said intruder, and believes the intruder to be a threat to life or property.
Now, him killing the guy with a sword, possibly taking at least two slashes (maybe one? Who knows maybe he had katana training) may be seen as gratuitous, and he may get in trouble for that, really depends on how well the opposing lawyer is.
I think he did the right think though, if I have a weapon, someone is in my house I do not know, my house had just gotten robbed, I warned the guy to leave or else, and he charges me, I would definitely defend my life even if that means killing him.