John Stuart Mill is an act-utilitarian and is most likely to support an act as right if it is likely to produce the most good for the most people.
On "don't kill", it would depend on the situation. If a lot of people would be happy from the killing of one person then this rule can be broken. If very little or no happiness would come from this then the rule should be followed. For example, letting a patient die because he/she is an organ donor and an organ is needed would be ok as compared to one person killing another for personal reasons between the victim and the killer.
Cheating would generally not be a good thing to do because it is usually for the happiness of one person. It would bring unhappiness to a lot of people so it is a definite no.
Not Smoking Tobacco is not a moral as even though it negatively affects you, those around you are not affected. Also, should one would have to make a count of the number of people that would be out of work if everyone stopped smoking, then the number of people that lose their jobs will be the majority and to keep them happy then we should have a choice or not rather than enforcing it as a moral to not smoke.
Whether it is right to keep a promise depends not only on the effect this has on the happiness of the promiser and the promisee, but also the community at large. That is to say, it is generally worth keeping a promise because it can be very upsetting and distressing to be deceived by one who is trusted, and (if the community finds out about it) it can make society generally less trusting and less co-operative. .
One must be very careful not to give one's own position favourable treatment - to not accept, as sufficient grounds for our breaking of a promise, grounds which we would not hold sufficient if someone else had given them for their breaking of a promise. .
That said, there are some cases where utility is better served by the breaking rather than the keeping of a promise.