Skarbrand cannot use command ability himself because he is not allowed to have the ability, because he is a unique character. He would require another demon unit nearby to activate the ability to place on him.
It is not true because named unique characters are not allowed to have other command traits and artefacts, not command ability. In cases of some unique character(as Aventis), their sub-faction is fixed so cannot take other sub-faction's command ability. But if a unique character have no fixed sub-faction, it will be chosen sub-faction of the army during the game.
Therefore, you can use 'Leave None Alive' with Skarbrand.
Khorne Command Ability Leave None Alive
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It is not true because named unique characters are not allowed to have other command traits and artefacts, not command ability. In cases of some unique character(as Aventis), their sub-faction is fixed so cannot take other sub-faction's command ability. But if a unique character have no fixed sub-faction, it will be chosen sub-faction of the army during the game.
Therefore, you can use 'Leave None Alive' with Skarbrand.