In this context, the pronoun "it" does not refer to "these games", but rather to the act of fighting and killing. The act is singular, hence the singular pronoun.

You could also word it like this: "these games make fighting and killing fun." You add "make it fun ..." in there as a short form of "make it a fun thing to ...". You can also use "it" in sentences like "it is snowing" or "it is sunny", where it's less obvious what "it" means. But I guess it means the weather. By the way - centenc -> sentence.