Cat loves carrier
My best friend and I discovered that cats respond differently to different types of carriers. I lent her my carrier to take her cat to the vet. We actually had to combine two carriers because the door on mine was broken. A few days later, she went to pick up the cat. The vet had lost the carrier! They gave us another one until ours was turned back in. The difference was almost night and day of how the cat acted in the carrier!
My/our carrier was black with a purple door. It was plastic and easily dissassmbled. (VERY HANDY!!!) There were "air vents" on the top cut into the plastic all around the top side edge ONLY. I had once tied a small string of balls and bells to the door. My cat loved it. She loved getting in and staying in the carrier. The first time I put her in it, I did it as a "practice run". I didn't go anywhere but put her in the carrier for short periods of time. I would then pick it up and swing it gently to get her used to that feeling. I'd put her down at any signt of trouble but left her in the carrier till she calmed down. Sometimes I would find her going into the carrier voluntarily!
I think the carrier being a dark color and very protected from outside view except for the door was a BIG bonus. The tan travel carriers with the metal tend to scare or excite them. I think it's too "light" for them in a stressful situation. The darker carrier seems to let them feel like they are "hiding" and calms them.
My suggestion for you right now, is to work on getting the cat to accept a carrier. Let it go in and out and get used to it. Then place that carrier close to the window where it may see that cat. That way you don't have to handle the cat but let it feel "safely" exposed to the other cat. The more exposure, the more likely the cat will calm down. It really is a terriotorial thing. The cat needs to learn to "shorten" it's terriotory a bit.
I would still take the cat to the vet to check for physical issues in the change of attitude. Maybe check into "declawing" if not already. I believe there are appropriate cases where declawing is necessary. In your case, attacking humans and NOT furniture is a good cause. Just get the front paws done.
Good luck! I know it's scary. My cat just bit me out of the clear blue the other day! She usually just "play bites" but this time she drew blood and punctured my arm! I think she felt bad about it later, and spent the whole day with me NOT allowing me to get away from her! She'd cuddle with me anytime I tried to get off the couch!
Scooby, Shaggy the "Dogs", Ms. Thang the "Cat" and introducing Measley Weasle "The Ferret".