I'm thinking of leaving a date with a really nice guy because I ran into an old flame
Are you dissatisfied with the guy you are on a date with?
No, it's just that I'm still have feelings for my ex.
You are not being fair to the person that you are on a date with. You should be nicer and kinder to people or you will develop a reputation, and others will not trust you.
I'm just realising that he is not the one for me.
I'm not sure who you are referring to here. If it is your ex who is not for you, then that is probably a good thing, since your relationship didn't work out the first time. If it is your current date, then it is okay to politely part company at the end of the evening, but I don't think you should just walk out abruptly, and certainly not for an ex.