Since purchasing and consuming animal products both depends on and contributes to animal agriculture, and animal farming necessitates rights violations against nonhuman animals, not being vegan when you have the option is synonymous with support for injustice. I believe that as leftists who purport ourselves to strive towards a more just society, and as human beings who generally value nonviolence and compassion, we should hold ourselves to a standard that doesn’t allow for the perpetuation of mass-slaughter. Let’s discourse!
But it’s not practical to sterilise hundreds of thousands of deer. It may be more moral to sterilise them than kill them. But you would struggle to implement that in anyway that could have an effect. The deer would reproduce faster than you could sterilise them without a cost in the billions. As well as that, reintroducing predators like wolves to a country isn’t as simple as dumping some wolves in a forest and hoping for the best, it takes decades of work to even get close to proper reintroduction of a species that was removed 500 years ago.