I'm going to cancel a date
Why would you cancel a date with someone?
I hate her and I'm not interested in seeing her
I'm sorry you feel that way. Cancelling a date can be a pretty unpleasant experience for both parties, but in this case it sounds like the best option. Hopefully it understands but you might not want to make dates in the future with someone you hate.
I have to cancel it, otherwise I'll have look at that one ugly trait she has.
If you feel such a strong dislike for this person why would you make a date with them? You might have gotten their hopes up when you knew it probably wouldn't work out. Having their date cancel on them could be upsetting and even insulting for them.