It's sometimes asserted that Mustafa Kemal (later known as Ataturk) regretted the Armenian genocide, but the evidence for this is extremely weak. Although he was not personally responsible for the genocide in 1915, he was responsible for preventing the survivors from returning, expelling those that tried to, and ordering his generals "to eliminate Armenia physically and politically" during the 1920 invasion of Armenia. (According to Raymond Kévorkian, only the Soviet intervention prevented another genocide.) Kemal's coalition during the war of independence relied on those that personally profited from the genocide and therefore had a lot to lose from Armenians returning and reclaiming their properties. Kévorkian has described this war as "intended to complete the genocide by finally eradicating Armenian, Greek, and Syriac survivors"—and he is not the only one of this view. In fact, Armenians continued to be expelled from Turkey for years after 1923 and to this day are generally prevented from retaining their culture outside of Istanbul (see Talin Suciyan, The Armenians in Modern Turkey).

Kemal's most notable statement on the genocide was in December 1919 to a domestic audience, as follows:

Whatever has befallen the non-Muslim elements living in our country, is the result of the policies of separatism they pursued in a savage manner, when they allowed themselves to be made tools of foreign intrigues and abused their privileges. There are probably many reasons and excuses for the undesired events that have taken place in Turkey. And I want definitely to say that these events are on a level far removed from the many forms of oppression which are committed in the states of Europe without any excuse.

Erik Jan Zürcher comments, "All the classic elements in the defense of violent aggression are here: they asked for it, it was not really so bad and anyway, others have done the same and worse."

Sources:

Kévorkian, Raymond (2020). "The Final Phase: The Cleansing of Armenian and Greek Survivors, 1919–1922". Collective and State Violence in Turkey: The Construction of a National Identity from Empire to Nation-State. Berghahn Books. pp. 147–173. ISBN 978-1-78920-451-3.

Ulgen, Fatma (2010). "Reading Mustafa Kemal Atatürk on the Armenian genocide of 1915". Patterns of Prejudice. 44 (4): 369–391. doi:10.1080/0031322X.2010.510719.

Zürcher, Erik Jan (2011). "Renewal and Silence: Postwar Unionist and Kemalist Rhetoric on the Armenian Genocide". A Question of Genocide: Armenians and Turks at the End of the Ottoman Empire. Oxford University Press. pp. 306–316. ISBN 978-0-19-539374-3.