Because it's not something we dig deep to prove. It's present in nearly every one of his films, and in BvS, there are numerous depictions of it without it being called for it. Watching the UC now, you had any number of hundreds of ways to establish that you are in a underground event for criminals. They decide to open it with a fight between some random white guy vs some random black guy, which is the one Bruce bet on and won. Violence is very much Snyder's thing. It's not even a bad thing, but it is what it is.
It's like asking to prove he has an excessive focus on violence is like asking for evidence of excessive slowmo in 300. It's fucking everywhere, and if you can't see it, you're either being intellectually dishonest or this is one of those debates where I have to explain every miniscule point with a goddamn dragon's horde of evidence because anything less is used to discredit the whole argument. The same way my disengagement of you is taken as admitted defeat, even though there is no real connection there.
If you are questioning something as axiomatic as Snyder's penchant for violence, then that's a clear sign this is will be a far deeper rabbit hole than I am willing to go down. No, thank you.