Basically, the second sentence seems horribly clumsy to me. I've spent so long looking at it that I have a mental block of how to change it.
So, an act could be seen as wrong owing to a different account of morality such as that of Kant’s or even utilitarianism. Even with this considered, Scanlon must accept that what follows from his explicit claim about what makes an act wrong is that wrong acts are wrong because they are unjustifiable to others.
The phrase even with this considered does not sound right at all to me but I can't think of how to express it differently.