My ex wished me Merry Xmas....

The least you can do is ignore him, given his past lyihg, abuse, and cowardice. Do't feel ashamed of anything you do to him after this experience. If you're nice, he will take that as weakness, and try to hurt you again, no doubt!

If you don't like him, don't talk to him. There is nothing good that can come out of talking to him. You will just wind up where you were before.

It's not a bad or good idea that you didn't say anything back. But it shows that you have inner strength for not responding and you're on your way to healing.

Just ignore him. He doesn't mean you any good.you are not obligated to respond
Also if he sent you a text, there is a big chance what you got was a forwarded text send to all the contacts in his phone

I know how you feel, my ex did this too. He broke it off with me saying some dreadful things like "I don't want anything to do with you anymore" and "these last few days have been the happiest I have been in ages" (we were on a break for the few days) so I assumed he was ending it for good. I was very hurt. Then on Xmas say he wished me a happy Christmas. In ways this made me feel happy he was thinking of me, but also awful because we started having a convo, and now these past few days he hasn't contacted me. I refuse to make the first contact. Maybe he will send you a happy new years message?

Maybe you could wish him a happy new year, keep it concise and amicable. "happy new year, hope all is well :)" or something like that. He will understand you needed your time to think things over, but are still thinking of him. I mean do you still have feelings for him?