Depends on what you mean by "selfish"
Selfishness itself is generally defined as being concerned, sometimes excessively or exclusively, for oneself or one's own advantage, pleasure, or welfare, regardless of others.
If that's what you mean by being selfish, then a simple act of helping others at the cost of your own pleasure, benefits, or welfare already disapproves that deep-down humans are all selfish. You can argue that such acts may ultimately caused by expectations of a perceived benefit, conscious or not. But consider these examples:
- A mother/father jumping in to save their child from a speeding car,
saving the child but killing them in the process.
- A soldier throwing him/herself to a grenade to save his/her fellow soldiers.
- Someone who attempt to stop a school-shooting, and got killed in the process
Acts such as those, which resulted in ending of one's life, most likely don't stem from expectations of future benefits (I mean, they would be dead afterwards, what benefits could they be possibly expecting?). And they surely don't have the motive of preservation of the individuals own life at heart.
Yes, you can still argue that "heaven" or similar religion's concepts may be the main motive. But I'm pretty sure that even people who doesn't believe such concepts would still likely to do the first example (at least).
Taking it further, such acts may indeed be explained by needs of preservation of human as a species (sacrificing one live in order to save more lives or future generation). If this still classifies as "selfish" to you, then I can't find other example. However, I would argue that what human behavior can't be ultimately traced back to "human species preservation" motive, with or without personal benefits? I believe that there's nothing wrong with that, and thus it's not a "dark outlook" at all.