What a beautiful kitty!
I doubt the dewormer had anything at all to do with this. Nothing. Throwing up blood can be caused by many, many things - tumors, for example - none of which you could have known about, or done anything about. One of my cats, Buttercup, died a few years ago after vomiting up something that looked like blood, and the vet said it could be ANYTHING. She had been fine the day before. As Xoostation wrote, you were following the vet's instructions. We depend on them to tell us what to do, and sometimes they are wrong. And our pets can't tell us what is wrong, either. Sometimes we know that they aren't feeling well, but they can't tell us why. And cats in particular are very good at hiding illness until it is too late. Do not feel guilty. You obviously love your cats very much and you do the best you can for them!
"Some of you say, "Joy is greater than sorrow" and others say, "Nay, sorrow is the greater." But I say unto you, they are inseparable. Together they come, and when one sits alone with you at your board, remember that the other is asleep upon your bed." Khalil Gibran