Major Rathbone's accomodation in Hannover

[My Name Is Kate]

I think it is very likely he would have killed them. He didn't show much reluctance in killing Clara, and almost killing himself.
There is no telling what was going through his head. He was certainly battling some some dark and evil thoughts, and he lost.

If he had just come clean right after the assassination, and admitted that he saw Booth enter the theater box (if he did, and from where he was sitting, he probably did), and the assassin would have had his gun drawn, so there would be no doubt of his intentions, maybe Rathbone wouldn't have had to keep piling lies on top of lies to save face and his reputation, until it drove him to criminal insanity.

(That's my take on it, anyway, FWIW.)