I am looking for an idiomatic phrase you would say to somebody you hate when something bad happens to them. For example, they could fall and break their knee or had bad news, etc. You wish them more of it, and you say it in their face.
Assuming your reason for disliking them is justified – let's assume this individual is a really nasty person who frequently belittles and berates others – then one idiomatic phrase you could use is:
Karma's a bitch.
If you wanted, you could even give this person the t-shirt.
If you want that person to experience something bad because they behave badly towards other people, then you might consider
taste of your own medicine
a sample of the unpleasantness that one has been giving other people
Then depending on what exactly you want to express, you can say something like
I'm glad you got a taste of your own medicine!
This means exactly that you are glad that this person also experienced something bad.
To imply that you want them to continue having bad experiences, you might be able to say "I hope you keep taking it!" But this is something I made up. I think it is understandable, but it is not commonly used.
There aren't a lot of set phrases for this in English. In my experience, this is less about what you say and more about how you say it.
Typically, I would expect this message to be conveyed with a relatively bland phrase and an acid delivery. In that context, any of the following would work:
It couldn't have happened to a nicer fellow
What a shame!
I'm so sorry to hear that!
You could say you gloated over your enemy's bad luck.
gloat - dwell on one's own success or another's misfortune with smugness or malignant pleasure
But note that most people aren't likely to approve of you gloating, and I don't think this would change if you found a different "synonym" (it's not a "nice" thing to do, regardless of what words are used).