I think the killer knew he would not be recognized (he didn't know Shinbach or the female patient) so in his mind they weren't that big a risk to him. They didn't know him. The killer lost control of the situation, whatever his original intentions were. I believe he intended to torture Faughey and either kidnap her or take the body out in the suitcase. I don't think he had a plan in place in case others were in the office. The robbing of Shinbach seems more of an afterthought to kind of change the motive of Faughey's murder. He may have wanted to confuse Shinbach & LE about whatever his original motive for murder was, and also he was pressing for time and knew he had to get out of there before LE arrived. The female patient who kicked him in the groin when he tried to push her into a bathroom, he knew he wouldn't have time to kill her. I think he knew they didn't know him, wouldn't recognize him; in his mind they were not a threat to him in that sense. Being pressed for time, he had to get out of there. His original plan, whatever it was, was thwarted with their being Shinbach and a patient in the offices at the time. The female patient, being a fighter and having kicked him in the groin; the killer knew that to engage in an assault with her would take even more of his time since she was a fighter; at that point he had to make his escape. In a fight or flight situation; due to circumstances he can't stick around and wait for the police.
This is a sick disturbed individual who built up a fantasy of Dr. Faughey (may she rest in peace) in his head, and obsessed over to the point where he came up with a plan, obsessed over that, and attempted to act it out. His plan appears to be torturing/murdering Faughey; whether in her office or not, i don't know. If he arrived there late at that time, thinking it was near closing, perhaps he hoped to get her alone and torture her there at night. I don't think he was aware of how many other people would be in the offices at the time, and that is where Shinbach and the female patient come in. The killer went in there and ended up having to "wing it", losing control of the situation and fantasy he had planned in his mind, resulting in the bloodbath.
This could pose a problem if he lives with others and thinks police are on to him, he may lose control again, so LE will have to work carefully with it.
Remember when he first walked in the office, he was calm and even chatted with the female patient waiting.
It's sociopathic behavior and very scary.