General Patton said far worse things. Furthermore, combat soldiers become really cynical about battle and about what civilians know about its realities, which in the case of Americans is thankfully essentially nothing. Mattis wasn't trying to reveal an eternal truth, he was speaking to military people who are used to that kind of talk. It doesn't apply to everyday life, and he did say "some" people. People who are oppressing the innocent and trying to kill you at the same time. Anyway, not many soldiers think killing is "fun," even though they are trained killers, and Mattis or a dozen Mattises won't change their minds about that.