I definitely think there is a correlation between meat-eating and violence.
That said, I don't think veganism is a panacea. It won't fix everything. But it will go a LONG way to creating peace on Earth.
Of course wars and murders would not stop suddenly just because people are vegans, as if it was a miracle, because people can’t all become true vegans overnight. It depends how the change to a vegan diet happens. More precisely, it depends if people are on a vegan diet or if they become vegans. Two different things, with two different results. If people are forced to be on a vegan diet because we abolish meat, but they still want meat, they may become even more violent. They will be on a vegan diet, but they won’t be vegans, and they might even continue to kill eachothers and still be on a vegan diet… unless they eat eachothers. After a few generations on a vegan diet, meat won’t exist anymore in the culture and customs of people, and maybe the ethical aspect of veganism will appear and then violence will slowly disapear. The change may come from outside (for instance, the government abolish meat, or, overpopulation overflows and the meat industry cannot provide flesh for everyone – but let’s hope they don’t turn to human flesh… which is very probable), and then people are forced to change. Or they choose to change by themselves, but this option may take much longer, perhaps hundreds of years.
But in both cases, at the end, the change must come from within also, it is the only solution.