As another user has said, maybe you have not look hard enough.
Originally Posted by Star_Jammer
"Genocide is a denial of the right of existence of entire human groups, as homicide is the denial of the right to live of individual human beings".
United Nation General Assembly Resolution 96 (I).
"Genocide is any act that puts the very existence of a group in jeopardy."
Henry Huttenbach, "Locating the Holocaust on the Genocide Spectrum: Towards a Methodology of Definition and Categorization", Holocaust and Genocide Studies. Vol. 3, No. 3, pp. 289–303.).
"The promotion and execution of policies by a state or its agents which result in the deaths of a substantial portion of a group."
Barbara Harff and Ted Gurr, "Toward Empirical Theory of Genocides and Politicides", International Studies Quarterly, 37:3, 1988.
"Genocide in the generic sense means the mass killing of substantial numbers of human beings, when not in the course of military action against the military forces of an avowed enemy, under conditions of the essential defencelessness of the victim."
Israel W. Charny, Genocide: Conceptual and Historical Dimensions ed. George Andreopoulos.
Any of this definitions could be applied to this Mutant extinction, at least according to wikipedia. Anyway, it's true that most of the definitions consider "the intent to destroy" as a requirement to speak about a genocide, not all but most of them. I will give you that. Maybe it's better to speak about an "involuntary mass killing"?
(I fell really stupid making this kind of distinction, but the truth is this would not be a strict case of genocide)