Imam al-Shafi'i (may Allah have mercy on him) said: "When expiation is obligatory for killing a believer in the land of war, and for a mistaken (killing) whose sin Allah, the Mighty and Majestic, has removed, then (expiation for) deliberate killing is all the more (obligatory)."
And he drew an analogy between it and killing game (in the state of ihram).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / Q16480