I'm not sure that any good could come from this. But it seems false to pretend I'm leaving for other reasons. How should I handle this?
If ever there was a way to burn your bridges, this is it. What do you expect to gain, other than possibly a sense of catharsis?
You don't have to give any reason for leaving, so if you can't find a positive one then simply provide the usual cliches about unmissable opportunities. Criticising your employer for things which could have been remedied, if only you'd asked sensitively, will come across as churlish; criticising for things that could not be changed will come across as petulant. Neither will leave them thinking well of you, and a bad reputation will spread through their networks - which may well include your new peers and superiors. Criticising your boss personally to his face will do all of the above, an order of magnitude worse.
The only answer you should EVER give for leaving is, "I feel this is the best move for my career at this time."
It may be entirely about working conditions, pay rate, policies, broken promises, or whatever else, but all of that really boils down to something that fits in the above statement.
You're not going to be the "Karmic Avenger." No one is going to have an epiphany because you left. No one is going to suddenly wake up and realize that they're the problem. They will go on, and if they truly are bad managers, they will end up with mediocre talent and their business will languish. You will find a job where you're appreciated and thrive.
Besides, if you do trash him in an exit interview, and he is a vindictive person, anyone you were friends with at your current job will suffer because he can no longer touch you.
Hold your head high, shake his hand, thank him for the opportunities and experience, and go be happy in life.
I left a job recently due to company policies and not due to my boss though this is a similar situation. These policies were in place due to maybe four separate individuals. I wrote a three page document calmly and efficiently explaining my issues with the company which I gave them in an exit interview. I did this not to change the company for myself, but to maybe change it for the future. I did not belittle, begrudge, or condescend to anyone, nor did I single out any individual; just laid out company policies I disagreed with and my reasoning why.
The company was very respectful of this and though they admitted to me that most of it was not going to change, they did accept it as feedback and constructive criticism.
I was also very honest with everything. I explained that I knew it wasn't my company and it wasn't my decisions. I explained that I knew other companies would have items I didn't care for as well, but that there were too many of these issues at my current job for me to continue employment.
All in all, I left on very good terms. I know this because after a few months off, I was propositioned to come back to the same company who offered a few concessions to me.
As long as you stay on the side of honesty and don't come off like a disgruntled employee who is just griping about your job, I think you should consider expressing your feelings to your employer.
Should I tell my boss I'm leaving because of them?
The answer here is unequivocally "no". Telling somebody above you that they are bad at their job is pointless. Especially when you are leaving.
The key question is
Should you tell other people at the company who care?
This is up in the air. Companies that provide exit interviews generally care about the quality of their people and they actually rely on your feedback to filter out bad managers. Have you ever spoken with your manager's manager? Do they know this is why you're leaving?
Without some feedback more people will be hired under your ex-manager and may suffer similar fates.
Don't jeopardize your future references on the back of a bad manager. If you are leaving because of your manager, ensure that you have independent good feedback from other parts of the company. This could be other team members or people from other departments who backed the quality of your work.
Ideally you get written references from these people before you leave (such as a LinkedIn recommendation).
EDIT from comments below.
If you are leaving because of a manager, there's a reasonable chance they don't like you either. Regardless of what you tell HR during your departure, you want to build a fence around that manager to ensure that people don't accidentally talk to them.
Allow me to offer a contrasting opinion. I absolutely value the feedback from my team members who are departing - especially when it comes to how I can be a better leader. Frankly, the exit interview is the time I expect my team members to be the most honest with me - especially about difficult topics.
Now, I've got very good relationships with my team members who have left and have spent their entire tenure fostering a culture where open and honest feedback is promoted. Did your boss do that? Can you provide constructive feedback? I can't answer that. I can't imagine many departures are that cordial.
But if my employee left and fed me a line of bullshit, I'd think far less of them (and provide less glowing recommendations) than if they provided open and honest feedback.
Worse, if I didn't know for sure why they left, I'd have to guess. If I guess wrong, then I may be "fixing" something that wasn't really an issue - only to make things worse for myself and those who remain.
So I would say to at least consider it. If you can provide open, honest, constructive feedback - and your boss will accept it, then go right ahead. If you have any doubts, err on the side of cordially fibbing through the exit interview.
I like Julia Hayward's answer, but here is a small argument for the other side: if I were your boss, I would certainly want to know that somebody found my leading so bad that he decided to leave the job because of it. It may trigger self-reflection resulting in me attempting to change my ways, and in the end, a better working atmosphere for others who stay in my team. Of course, there is no guarantee that I will actually attempt a change - I might dismiss your opinion as wrong - or that I will succeed if I attempt it.
But if I were the person leaving, I probably wouldn't take the risk in most cases. The chance of actually achieving a change depends a lot on the personality of the boss and their preferred ways to deal with the cognitive dissonance caused by you pointing out that for you, their leadership was badly executed. You may want to try it only if you have the impression that the boss is a person who welcomes criticism and doesn't overreact even when he/she judges yours to be unfounded.
But even in this last case, there comes the question why you didn't try to talk about your differences during your work for them, rather than to wait until leaving. If you judged your boss to be likely to listen to your criticism, you should have expressed it earlier and helped them change. If you are sure that they are the person who would just deny everything and hate you for having a bad opinion of them, then there is no sense in talking to them about it at all.
There is no need to explain things which are no longer necessary. Since you are leaving your job, thank your boss and colleagues for their support and help. You never know you may need their assistance in future. Must remember that the needs of any boss can change as the business climate does, and the good rapport you once had with your boss will give you future benefits.
When leaving a job I think that it is not fair to be dishonest and avoid the real reason why you made the decision to resign. For example if you are asked to to complete paper work during your shift and your employer has not provided time for you to complete it during your shift then they are being dishonest. If your employer will not accommodate you with scheduling, but makes accommodations for others.
There are lots of good answers here explaining why not to do it. In some cases, it will help the company you're leaving, but it's unlikely to help you personally and may well hurt you.
If you feel like you need to do something, my advice is to write a letter on your home system explaining why your boss is such a problem. Spend some time on it. Once you have it finished, leave it in your system so you can reread it now and then. (Alternatively, you can delete it, but that dilutes the effect for me.) Never send it to anyone and never show it to anyone. In my experience, that takes care of the emotional reaction well enough, and letting anyone else see it is much more likely to hurt you than help you.