okay i'm watching chicago med and some ppl are saying that sarah reese is the most irrational and irresponsible doctor in the series but i don't really agree
when she pepper sprayed a patient, she obviously felt threatened. she was shown to be increasingly unstable up to this point, something she refers to as PTSD in the following episode. the patient she was dealing with was being aggressive and literally backed her into a wall, there was no one else there and she was scared with no other way of defending herself. she asked him to move away from her multiple times and then he didn't, so she had to do something.
her paranoia also comes from dr charles - another psychiatrist in the hospital that she's very close to - getting shot by a patient, so of course she'd be scared by a patient being aggressive towards her.
like obviously a doctor shouldn't be pepper spraying a patient but there's other doctors in the series that did things that are a lot more illegal and irresponsible