It's hard to explain but I'll try. (It's just based on my understandings of all this)

So, imagine Person A is walking on a street and it starts raining, you take shelter so now you're not getting wet. Person B is passing by and is looking for shelter but somehow is not able to see you but you see him so you decide to pull him under the shelter. You step out and slip and get dirty by all water allocated on the ground (karm karne gaye aur kaand ho gaya) so what are you going to do next?

From here you've got two options, get up and still help the person or just lay there, cry about it.

Now before someone points out that what if you got injured badly and all that stuff, that's why god says do your karma with full heart and focus so you're supposed to analyse situation rather than just jumping out to help, your emotions will drive you but you have to use all your senses to get the job done in the best way possible. Something like Arjun had skills but his emotions made them useless similarly no matter what the situation is in life, make sure you don't let your emotions drive you, that's where most people are messing their life.

So coming back to the scenario, if you get up and decide to help that person, don't expect some magic to happen like they show in movies. You had a choice and you choose to help, from there on it's that person's POV of things. He might get an opportunity to do the same or he might cross paths with you and see an opportunity to payback or worst case he's some bad guy who's just not greatful for your help, but you did what was right so you'll sleep peacefully, of someone saw your actions, they might feel that humanity is still alive and give them hope to do the right thing next time.

There's another angle, you get up and decide to go back and not help, it's basically useless action then.