Can Someone Explain?!

I'll start from the beginning I guess. My mom had the 55 gallon that I currently own, before me. She bought an Synodontis Catfish. She bought him about 4 years ago, when he was maybe an inch long and he was in the 55 gallon with Convicts, Jewels and Silvani's. A fairly aggressive tank. Over the years the Silvani's died.
In August she gave me the tank and the catfish was the only one to survive the trip. After figuring out what he was, an Ocellated Synodontis, I read he was a community fish. Cool, I wanted a fantail goldfish tank. So I put in two to test it out, and everything was great. He actually even came out and swam with them. You could even see a couple battle wounds he had from the cichlids, poor thing .
Fast forward to a couple days ago I had six fantails and one comet/common goldfish in with him.
Today, I literally just came home from work and two were picked at until dead! Like actual chunks missing out of them. I've had the same stock in there for the last three months. He was still out and swimming with them. I never saw any signs of aggression or chasing behavior.
I put in algae wafers and carnivorous sinking catfish pellets. Which he always eagerly ate. So it's not like he was starved.
Why did he suddenly kill two of his buddies?!
He's presently being acclimated to my 20 gallon which has an aggressive Black Asian upside down catfish, but that's literally the only choice I have until I can get to a pet store on Friday. I don't think both are going to live that long, but I don't know what else to do.

 

So sorry to hear about your goldfish. Just how big is the Ocellated Synodontis at this point? What size were the goldfish? If he's very large, and the goldfish were small, he may very well have decided to make a meal of them. Generally, I wouldn't recommend putting goldfish in with tropical fish. Besides, goldfish are messy and need huge water changes, and Ocellated Synodontis like very clean water. Perhaps you could sell him and use the money to focus of goldfish if you like them more.

 

He's about five maybe six inches and the two he killed were about four inches. The thing is was that it was the like mid-sized ones. Not the biggest two I had, or the smallest two. I added them in pairs and I love to watch them grow.
And that's kind of my plan for him is to take him to the shop for credit to put toward new fantails. But I get the feeling the two catfish are going to fight in my 20. I feel so bad but I don't want anyone else to die. :'(

It just confuses me how he all of the sudden decided to kill them. I do water changes every week. I keep the water pretty clean..

 

Are you sure the goldfish didn't pass before they were snacked on? (They also could have been picked at but the other goldies as peaceful as they look they can be aggressive as well),one day together likely won't have someone killed, however if you need to contain one inside a large bowl floating in the tank.

 

One of the goldfish was still alive (barely). So at least one of them was alive when attacked. The goldies all swim together and have never shown aggression. The last two were added almost two months ago. So its not like I added a new buddy and they attacked.