Whenever you suspect or know of a nurse who may be (or actually IS) mistreating patients in his/her care, it is your responsibility to report the mistreatment, but I would FIRST speak to the nurse in question about your observations or patient comments about a particular nurse BEFORE writing him/her up.
Secondly, if your talk with the nurse is "ineffective" and you still notice the nurse mistreating his/her patients, let that particular nurse know that you are going to request a meeting with the Nurse Manager so that the THREE of you can discuss your findings, etc. If that meeting does not effectively improve matters, THEN let the nurse know that you are going to report his/her mistreatment of patients to your Nurse Manager's Supervisor, as well as your hospital's Patient Representative Office (if your hospital has one of those).
I agree that you should not ignore your findings of a nurse who is not behaving professionally to his/her patients, but I also think it is a common "professional courtesy" to give fair warning to the nurse in question. I would NEVER write someone up without first having a talk with that person about their behavior, etc.
In the future, perhaps you can enter your assigned patients rooms and say "Good morning Mr. or Mrs. or Miss or Ms. "B", my name is enurse and I'll be your nurse today. How was your night last night? Did you rest well? Is there anything I can help you understand about your treatment during the night or what we will be doing for you today?"
There are ways of "hearing" how your patients previous shift care was performed without mentioning anything in reference to the nurse who cared for your newly assigned patients during the shift before you came to work. Professional thoughtfulness and clarification goes a long way to improving the workplace and peers you work with. Of course there will always be those cases that will not cooperate, no matter how much "room for error" you give them, but always try to at least extend that professionalism no matter how you are treated. There are ways of seeing discipline in action with those employees who continue to act unbecoming as nurses while at work and in the care of patients.
Best of luck to you, enurse. You are on the right track, and I'm sure you handled the situation as best you could with your current nursing experience, so don't beat yourself up over this. We have ALL mishandled a situation where down the road we come to learn much better ways to deal with matters like you are dealing with. Sometimes a nurse acts out for attention, and by your bringing his/her actions to the forefront in a private conversation with them BEFORE involving the Nurse Manager or others, is all that person may need as a reality check. You are young now, but one day you too may be a sixty year old nurse who is fed up with "whatever". Also, "bad attitudes and unprofessionalism" are not just characteristics of the aging population, but also of many young people.
Of course, there are NO excuses for mistreating patients, so don't think I'm making any excuses for that nurses behavior because I'm not. I'm simply offering you another possible way of handling problems like this in the future because "there WILL be others"!