Atheism and Ethics

[Angrboda]

As an anti-realist, one could behave morally, but why would one want to do so?

Why would one not want to? We have evolutionarily guides desires and wants that are moral in nature so we tend to follow those. Also, we do an awful lot of things despite there being no objective standard or goal behind the actions. The claim (not sure if you are implying it through your statement) that we wouldn’t want to act a certain way without an objective standard I think credits us with being too rational, that we analyse our actions and only proceed if certain criteria are met

There are plenty of incentives for behaving immorally. That's why.