Sometimes we really need to wait to write a letter of complaint...this is one of them.

Had you waited and written this a few days afterward you would see that you over reacted to this incident.

This employee did two things for two customers...she was going to take the cart back for you and then give it to another customer waiting for it.
If anything I would be thanking her for what she did as it saved you a trip to the cart corral.
Instead you acted as if she was stalking you throughout the store and then pounced on your cart before you could get your groceries out of it.

I would think seriously about recinding this complaint and instead write a letter saying you over reacted and realized that this employee was instead giving good customer service.