If you are someone who likes to help others, is helping them actually a selfish
In the general muddle of psychological impulses that might come under the category of motivations for a given action, we can distinguish between our principle aim(a) in doing the action, and enabling conditions such as its being broadly in their interests to do such actions. The mere existence of such enabling conditions does not mean that they figure in one's principle aims; the mere fact that it is in my interests to look after my child does not mean that that is my principle aim when I treat her kindly - in particular, it does not mean that my interests are what I have in mind when I treat her kindly. So one might have a situation in which someone - a nice person who enjoys helping others - has nothing more than 'helping my friend' as her principle aim, even while something like 'I'll feel better for doing it' might figure as an enabling condition (it might make it easier to put in the necessary time and effort that the friend needs).
We judge people in important part by reference to their principle aims, and if someone's principle aims are sadistic then they are morally speaking entirely different from someone whose principle aims are to help others. Kant thought the mark of morally good action is doing it from duty, doing it just because it's the right thing to do. He thereby sets (I would say) a very strange standard of moral worth; one which has no place for altruistic feelings as moral motivations. By contrast, Hume before him was more Aristotelian and conceived most good moral actions to be, simply, those that we naturally admire. Here, in this (I would say) more natural philosophical conception of the moral, we find a proper home for the idea that if a person's principle aim is to help someone, then they and their action are to that extent morally good. The sadist's acts of sadism have no such admirable motivations.