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
Sayyiduna Wathilah bin al-Asqa' (may Allah be pleased with him) said: We came to the Prophet (peace and blessings be upon him) concerning a man who was a companion of ours, for whom the Fire had become obligatory. He (peace and blessings be upon him) said: "Free a slave on his behalf; Allah Almighty will free, for every limb of it, a limb of his from the Fire."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16480
This has passed before, except that in this narration he said: "concerning a companion of ours who had made the Fire obligatory for himself through killing."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16481