When you kill a magma cube it doesn't split into more magma cubes.
Comments 3
Well, back on June 8, 2015 (when that comment was posted) Magma Cubes weren't splitting when killed. This feature wasn't added until 0.12.1 build 1. see http://minecraft.gamepedia.com/Magma_Cube#History
Now (versions 0.15.90.8 or 0.15.10) this should be working fine. If it is not, please create a new ticket.
At this time, Magma Cubes have not been fully implemented, so this works as intended.