is this right?

That depends a lot on how it is hurting others...physically or mentally? And in what way? Is it only temporarily hurting them and will they be better off in the end? It really depends on these things and what you define as the right thing and happiness. It might be that this thing you do will bite you back in the end and make you extremely unhappy. This is as very situational question. In the end, you're going to have to weigh the variables, make a decision, and deal with the consequences as they come. BUT if it physically hurts people, then you are dealing with a bigger bird...

I think that you have to think about everything that might happen if you choose to do something, and then only you will be able to make the decision with regards to whether you believe that it is worth the risk or not. If it is something selfish that you want to do, then you should probably think twice about it, however if you believe that you have to do something for the good of the long term then you should probably do it, even if it hurts people at the time, as long as you truly believe that they will thank you for it in the end.

That's too vague to answer. Is it legal? Right? Justified? Long-term? Reducing your unhappiness? Temporarily making them upset? Damaging their lives? Deliberately sadistic?

Happiness will be more enjoyable, if not make others hurt. Happiness is a perfect, if we can be happy, and make others happy.