It's not so much being a sadistic maniac, as much as it is that the bystander effect is absolutely a real thing - until you're actually in a situation it's impossible to know how you'll actually react to it, and a lot of people in high stress situations freeze up unless called on specifically.

It's hard to say how weird it is to feel like you have to take action in a situation because you know that nobody else is reacting.

And regardless of how much it seems like the obvious thing to do, I'd argue that saving a life that would have otherwise died is what a hero is. Saving a drowning person, even a child, isn't without risk, and most people don't really know what to look for with drowning (it's not always the big thrashing obvious mess - you sometimes have to actively notice a person go under and not come back up) to begin with.

It's not just the impulse to do the right thing, it's also having the means and knowledge to take the correct action that makes you a hero in these sorts of situations.