I would like to say several things:
One, please provide more specifics on your situation - fish lost, how old they were, and SPECIFIC water parameters - if you are truly interested in ruling out any other possibilities. And there may be other possibilities. It is worth making sure there is nothing else that is an issue.
The brittlestar is a scavenger...and will eat dead fish. But whether it caught and ate those fish is questionable. How many fish did you lose? How often did you lose them? Did you ever see the star with a huge disk? Again, important to know what fish, how big, how long you had them, etc. Because many of these reports of brittlestars CATCHING fish are WRONG.
In short, from what I can tell this was an animal that was not directly fed. This is important for animals we keep and I can not blame it if it "decided" to get some food. To expect it to starve is rediculous...however, I would imagine that you would have had issues for quite some time if you had a predatory star. If this was a recent thing, you may just want to rule anything else out.
But I like to document these things so I would very much appreciate specifics if you could provide them.
The green brittlestar is a KNOWN predator in the wild. But many brittlestars have the potential. They are highly adaptable and opportunistic; deprived of food, many could easily turn to getting it in other ways.
I agree that the sand sifter is doomed in most tanks under 100g and will generally starve in 9-12 months.