The suggestion that life has no meaning is a confusion of scaling.
Are you an animal in a circular ecosystem? If you're human you can check that meaning box as 'yes'. Bees, bats, birds, and other flying pollinators might have a more direct line into our thinking with that but there are things that would go haywire without any given species. One can argue too many of anything is a bad thing - that's a slightly different issue.
Are you a person in a political and sociological ecosystem with some sort of belief that's part of the blend? If so you can check yet another box of meaning as 'yes'. Your opinion could be woefully ignorant or misinformed but it's one tick in an ocean of many ticks that democracies rise and fall on.
Do you interact with other people and affect their lives? Yet another box to check 'yes' for meaning.
Can you feel physical anguish, emotional anguish, or both? If so that's yet a third meaning box you can check as 'yes'. If you're in a situation where pain is being inflicted on you directly by what's happening around you or to you - life gets really meaningful really fast.
The cosmos might go on just like it ever has if Earth looked like Mars within 1,000 years but again - scaling.
On a different note, with murderers, some of them indeed did have nihilism in their variety basket of reasons why they did what they did. Most of them also had a lot of other things in there too - ie. abused as children, ostracized by peer groups, mental illness, brain damage from abusing the right drugs, brain-eating STD's, etc. etc. You have people who've either been treated so badly or who've been so disaffected by what they've seen that they want to not just watch the world burn but do what they can to participate in it burning sooner rather than later. In a way they've found a purpose, a diabolical one, and it doesn't quite map on to nihilism in the usual sense.
So nihilism is clearly destabilizing and unhealthy, also on it's own its more likely to just make people less happy, less healthy, eat away at reason and common sense (eg. post-modernism), and make the background energy of society more sluggish rather than raising the murder rate a whole lot. That said we have lots of reasons to beat nihilism for its vampiric effects on happiness, reason, and common-sense before we get to the issue of murder.
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The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.