If you use "/kill @e[type=!Player]", this is supposed to remove every entity from the map except for the players. But if there are slimes or magma cubes, they will simply split into smaller ones, so you actually have to use this command thrice to effectively wipe them out for good. I consider this a bug or at least an oversight because even Enderdragon is killed by this command without side effects (creating an exit portal).
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So why enderdragon is not normally killed, but just disappears?
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(Assuming the command doesn't just make entities disappear, but cause a normal death...)
The normal death of a large/medium Slime results in smaller Slimes spawning. Same with Magma Cubes.
The Enderdragon bit might be a bug, if you expected it to do the entire death sequence. But if you spawned more Enderdragons, this could be annoying if you had to remove portals every time you ran the command.
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And slimes splitting instead of just disappearing is annoying too. That's the point of this issue report.
No it's not supposed to remove every entity from the map. It's supposed to /kill as the command itself is named. You are killing the slime so it splits into its counterparts. Works as intended.