OP, so very sorry that this has happened to you. and with Patau's tbh it's basically not a decision to end the life of the baby, it's actually doing the kinder thing for the baby by ending his life before he has to face a world of discomfort and pain and then sadly die anyway. You did the right thing for your boy.
Kate I know you just blurted out what was in your head at the time and I have done the same, many a time on MN and regretted it so you're not alone in that!
There are lots of women on MN who have had a baby with DS and who love their children very much. And I have a cousin with a beautiful boy with DS. He is the light of her life. But he spent the first three years of his life in hospital, has had numerous heart operations, has never eaten any food orally, so has a tube inserted into his tummy in order to feed. She adores him and I honestly don't think SHE would make a different decision now if faced with the same choice again, nor would anyone in our family wish she had chosen differently but the sacrifices she has had to make have been huge. You honestly can't berate someone for making that choice as sadly, it isn't always just a case of being a lovely child with DS, they all are, but for all you know, they may be facing the same life as my cousin and I wouldn't blame anyone one whit if they thought that might be too much for them. My cousin is a wonderful devoted Mum but she has had very, very bad days with her boy.
I have an aunt also who lost her son aged 5 from complications from heart surgery which sadly a lot of children with DS will need. I have also lost a child and the pain of that is something I wouldn't wish on anyone. If you have a child with DS, you are much more likely to have that child die before you than if your child didn't have DS. Do you see that it's not as simple as you originally thought? And that it MUST be the choice of the couple or indeed the woman who is pregnant herself to make the choice about what she can and cannot take in her life?
I do think you sound like a lovely person and your recogniition that people with DS are human beings and deserving of love and respect is commendable but the Antenatal Tests/Choices topic can be an awful place for an OP to find themselves posting in so it's best to be mindful of that.