Specifically, each and everytime they justified war. I'll even spot you Afghanistan, as the declaration was justified, though the method wasn't (fuck hearts and minds, just go there, kill the leaders, make it clear you'll leave the second you have them, then actually leave the shithole once they are dealt with).
McCain supported the Iraq war and still continued to, and not only that but was a leading proponent of it prior to it's beginning (for example, on an October 2001 interview with Letterman, he called Iraq the second phase of the war in Afghanistan); hence, I find him to be scum.
Meghan McCain, while not having hard political power, still cheerleaded the wars, including calling anyone against war in Syria Assad apologists (Tulsi for example), and condemned Trump and Rand Paul for supporting removing troops from a war we had no business in. I've little doubt I can give more examples for other wars if I wanted to spend the time to look.
So you just broadly say every time they justified war...but then immediately "spot me" with Afghanistan and admit that declaration was justified. That opens the possibility for other declarations to be justified, too. So you can't just broadly say all the wars they supported were evil by default, you have to criticize them case by case.
When it comes to wars in the Middle East, and military action around the world in general, I think libertarians have a rather naive notion that if we would just leave everyone alone, everyone would leave us alone. That hasn't been true at any point in human history. It would be nice if it were true, but if all nice things were true things we would have a cure for cancer, Elvis would be alive, and the Star Wars sequels wouldn't exist. Nations have always been getting in their neighbors' business. And the more powerful and prosperous a nation is, the more they become a target. Libertarians also like to ignore the existence of violent ideologies that are going to hate us regardless of what we do.
We saw the result of such ideology on 9/11, and we took steps to remove the threat. But I also think it's naive to act like it's a one and done thing. The United States needs to show constant vigilance on the world stage, constant action to fight terrorist groups like Al Qaeda and prevent them from establishing bases and building a capacity to train forces that can then be used to attack us. It's naive to think that it's all fine now, we can all go home, 9/11 can never happen again. A naivete that could cost thousands of lives. I have zero confidence that after the fiasco of the pullout from Afghanistan, the country won't go back to being a safe harbor and training ground for terrorists like it was 20 years ago.
Libertarian criticism of the US for war is also historically shortsighted. The US military is one of, if not the most, conscientious in history. Far from perfect, but in the grand scheme of things, we do a lot more to try to minimize civilian collateral. Rules of engagement, billions of dollars spent to develop ridiculously precise weapons systems.
And to that point, there's the reality of rivals like Russia and China. The more weakness the United States shows on the world stage, the more Russia and China will assert themselves. You cannot tell me that Putin's orders to go ahead in Ukraine were not in part spurred by the US humiliating itself with Afghanistan. You don't like death and mass killing? I don't think you want to find out what would happen if Russia and China knew they could throw around their military might and never have to worry about the US getting in the way. A friend of mine, retired marine, once said this to me - having the biggest gun in the world is worthless if your enemy knows you will never use it.
The evaluation of any given war and how necessary it is to securing your own citizens' safety can be a flawed judgment. You can have bad information going in that makes the war seem foolish in hindsight. War is awful and traumatic and deadly, but it's also a reality of history, of humanity, a reality that we have to deal with or it will deal with us. So, political leaders ordering military action to protect our interests, to ensure the security of our citizens for the reasons indicated above, can be valid, can be just. Again, I think it's naive to accuse leaders like John McCain of being "scum" for having that sort of reasoning. Maybe he was wrong in his evaluations, with tragic consequences. But that doesn't make him a monster deserving of you dancing on his grave and spitting in the face of his daughter.
As for my standard of evil: war is evil. War is a mass killing, and largely innocents die. It is one of the biggest violations of the NAP there is for how many times it will cause individuals to violate it. Sometimes very rarely war is a necessary evil (which is why I spotted you Afghanistan at the beginning), but joining a war when you weren't attacked makes those times vanishingly small. Iraq and Syria and Lybia were not justified. We had no business being there, they were no threat to the US. So I condemn each president that had a role in those, and hate each one (also the US support for war in Yemen).
But here's the question, how is any of that connected to a concept of "evil"? In your own words, can you explain just what the deaths of innocents, what violating the "NAP", has to do with evil? I have my own answer, at least with regard to the death of innocents, but my understanding is that you have a different moral framework than I do, so I'm curious as to what your own answer would be.