a couple people expressed some interest in this so I'm going to try noodling on it in a more coherent fashion
The first thing is that - as other people have written about - when constructing a modern AU it is important to think about how characters in a historical or fantasy setting relate to violence in a proportional rather than literal way. MDZS has a setting where killing is more normalized than it generally is in modern society, both on a personal and a societal level.
(This is also why acting as though a fantasy villain blowing up a planet is equivalent to the concept of a real planet being blown up is silly on a level that's not just the equation of fictional and real; it's also a matter of scale in the setting or genre.)
So looking at Xue Yang's propensity for violence in canon as a way to consider how he might behave in a modern setting firstly needs to examine the ways in which that violence is calibrated to his canon setting.
It is definitely true that, even relative to other characters, Xue Yang's violence is marked as beyond the pale and extreme. So it would follow that the same would be true in a modern setting - but my argument is that it's not quite as straightforward as that, because there are certain contextual things about Xue Yang's violence that I think are important to take into consideration.
The first is to note that his most notorious act of extreme violence - the massacre of the Chang Clan - is one that occurs while he is under the protection of the Jin Sect, and he knows it. (In CQL, he gets permission from Wen Ruohan, though it's true that he never invokes that as a defense.) There is every indication that Xue Yang is thoroughly unconcerned about the prospect of getting in trouble for the massacre. Xue Yang isn't acting without being mindful of potential consequences; he's acting in the knowledge that he won't have to deal with them. The second is contextual: who Xue Yang chooses to target, and where. For instance: the people he tricks Xiao Xingchen into killing live in a remote area where he's a little risk of authority intervening. In both cases, Xue Yang is acting under conditions where he's unlikely to face consequences; where his violence is, if not considered acceptable, then something he can get away with.
Xue Yang can be reckless, but that recklessness is tempered by a very strong survival instinct and a recognition of what he needs to do in order to stay free and alive.
So then, to carry this into a modern context, particularly in a setting with a state-sponsored police and a defined legal system: I think that consciousness of the risks he'd be taking with acting violently would be even more acute, particularly because it's likely that he would be existing in a state where he'd come into contact with the legal system early for more minor crimes. Connected with that, there's the fact that the tolerance/acceptance of authority for violence outside of warfare or state acts is significantly lower, so any shielding he might have for acting violently illegally would be much thinner. Xue Yang is aware of the extent of what he can get away with, and "what he can get away with" is less, under a modern legal apparatus, than he could in the decentralized jianghu with its ad-hoc justice system.
When I say that I think Xue Yang's violent tendencies would be tempered by the existence of a modern legal system, I'm absolutely not saying that it's out of respect for that system. (I would hope that'd be obvious.) And I'm not saying that a modern Xue Yang wouldn't still possess violent tendencies (I think he probably would). I just think he's highly motivated to consider the context and targets for his violence in such a way that would keep it much less visible and extreme than it is in canon. There are still acceptable targets. But he's not going to kill a whole family. And I actually think he's likely to stop short of murder in general.