What if I don't let a person go in front of me?
Why would you not help the person?
They didn't let me go in front of them.
It isn't expected that you give up your place in line. Since they didn't give up their place to you, you shouldn't be expected to give your place to them.
I hate when people do that and I hope that they die.
Did you not just say that you didn't let someone go in front of you either? Hoping that they die would mean that you also hope that you die, and that's not right. Just keep your spot in line, but don't hate others for trying to get ahead as even you would do it too.