So let's say a Dalish elf believes that humans are the cause of elven mortality, and he wants to save his people and make them immortal, so he does everything possible to cause a total and complete genocide of humanity. Disease, massacres, forced sterilizations, mutilations, etc. Hundreds of thousands of people are killed in terrible ways.
Is this person good, because the intention - to save the elves and restore their immortality - is a good intention?
Moreover, your characterization is problematic because we're not clear on what intention we're talking about. Clearly someone intends their means as much as their goal. So if my intentional goal is good - e.g. save the elves - but my intentional means are clearly evil - commit genocide - then which intention trumps?
Doing good for your people is generally regarded as moral. Helping them, taking care of them, putting their safety and happiness above yours. Who you consider as "your people", that's up to you. If it's "all living beings", genocide is amoral. If it's a smaller group, killing people outside of it would not be seen as amoral, or at least less so.
Whether the end justifies the means, that's a good question. Does it depend on the means and the end? Was killing Orlesian chevaliers to free Ferelden good? If the warden kills a guard who's just doing his job to infiltrate a mansion, is it an evil action? If someone kills a friend or a family member, would it be evil to kill them too? When something is personal, people stop caring about the reasons behind actions. To put it coldly, there's a conflict of interest. If I were a human in the scenario you presented I would try to stop that elf, and probably end up doing some questionable things in the process. But I don't know if I would call that "good".
Morality is a dodgy and complex subject. I still don't have a defined opinion about it. My current take is what I wrote earlier: "From a philosophical point of view, I'd say evil people don't exist. Everyone is merely the product of their life experiences, and their own reactions are derived from these experiences too. From a practical point of view, there's a point where you have to draw a line, stop thinking about people's abusive childhood and plant a bullet in their head."