They would both be attacking, yes. Master of Cruelties has a restriction which modifies the declaration of attackers, but if he is put into play attacking with Alesha, Who Smiles at Death, Kaalia of the Vast, or some other effect, it it already past the point in combat where that ability is relevant.
April 15, 2015 10:25 p.m.
Master of Cruelties's third ability only prevents you from declaring Master of Cruelties as an attacker alongside another declared attacker. Because you're putting Master of Cruelties onto the battlefield attacking, you aren't declaring it as an attacker and its ability does not apply.
Its last ability will still trigger even though the wording "attacks" normally requires a declaration of attackers. There's a special definition for "attacks and isn't blocked."
4/15/2013: An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.
April 15, 2015 10:26 p.m.
Furthermore note that the Master of Cruelties's final ability will also still trigger in such a case if it doesn't get blocked.