I wouldn't worry about your feelings toward working with offenders. Whilst in principle it sounds morally difficult, the reality is far easier to deal with.
In forensic psychology a lot of the people you work with are people who are mentally unwell, and may have committed their offence due to being unwell. This certainly dilutes down any malice you feel toward them as they become patients rather than criminals.
The next thing is the difference between a piece of paper and the person in front of you. You don't see the person for their crime, you see them for the individual in front of you. Actually you can form good relationships with offenders and their crime seems somewhat distant from the reality in front of you; they can be genuinely nice people. You see them for who they are and not what they've done.
Finally very few criminals are genuinely psychopathic merciless criminals doing it for a laugh. People commit their crimes for many understandable reasons e.g. being unwell, self defence, revenge, intoxication, needing money, terrible social circumstances and life events etc. Whilst this of course does not vindicate what they did, it allows you a degree of understanding which in turn can soften your opinion of them.
Of course you will meet genuinely nasty people who have done dreadful crimes for poor reasons, but they are very much the minority. Then it is simply a matter of being a professional and doing your job, regardless of your personal feelings.
In terms of myself, I qualify as a doctor next year and am currently doing my elective. This is a 6 week period where we can do anything we like (medically related) anywhere in the world. My day consists of;
Going to jail and assessing patients
Seeing patients on the ward at the secure hospital I'm based at
Seeing outpatients who have been discharged from the hospital and are now in the community i.e. visiting them in their homes
Attending the multidisciplinary team meetings
Attending the teaching and academic programs they put on at the hospital
I did a BSc an MSc and am now obviously doing my MBChB, but a standard psychology BSc is enough to qualify someone for a graduate entry medicine program. Medicine obviously opens up over 70 careers and you can become anything from a GP to a surgeon. I'm interested in pursuing psychiatry as a career although I haven't fully made up my mind.
Forensic psychology is a great career but, as with all professional psychology jobs, is dreadfully competitive to get on to. Medicine is also competitive to get in to, but forensic psychiatry is a far easier career (psychiatry is massively undersubscribed) to get in to once you overcome the hurdle of getting into medicine. It also has far better job security, is better paid and is generally a more senior position within the forensic team (made up of OTs, nurses, psychologists, social workers, support workers, doctors etc).
Sadly due to the delicate and secure nature of the field it is often difficult to get direct work experience in the service unless you are a medical student. I was discussing this with the consultant forensic psychiatrist who is supervising me. He says that you have to go through far more hurdles in terms of trust permission if you are non-medical.
In lieu of that difficulty I'm happy to answer any other questions you have about forensic services, the roles of doctors and psychologists etc. Hope that helped.