That looks like a road kill cactus-- the common name. It prefers more water and humidity than the average arid desert cactus. Looks like the cactus is dieing back in the first picture because of less than ideal growing conditions. I see the same issue in the second picture plus the plant was leaning a bit before it fell over more.

Also it looks like you just repotted it, is that correct? The pot looks oversized relative to the plant and with that type of soil mix in a pot that large, you're going to have trouble watering it correctly. If you soak it all the way through, most of the soil will stay wet for a long time, potentially leading to root rot. If you just water it sparingly, it's not likely to be enough water. I'd recommend either 1) going to a smaller pot, at most half that size, or 2) switching to a gritty mix comprised of things like pumice, perlite, Turface, etc.

See this thread: Water movement and retention in container soils