I was confussed something Arminius said in his response to Perkins so I looked it up in the original Latin. Turns out it was a mistranslation.
In the former case [the creation of animals, plants…], the good communicated is limited, as each creature receives that which is appropriate to itself, according to the diversity of their natures, but, in the latter [the creation of men and angels], there is a communication of supreme and infinite good, which is God, in the union with whom consists the happiness of rational creatures. Reason demanded that this communication should be made contrary to justice, wherefore He gave a law to His creatures, obedience to which was made the condition on which that communication should be made. Therefore, this was the first decree concerning the final cause of rational creatures, and the glory of God to be illustrated by justice and the highest goodness — highest as to the good to be communicated, not absolutely; by goodness joined to justice, in the case of those who should be made partakers of the highest good, through steadfastness in the truth; by punitive justice, in the case of those who should make themselves unworthy of it by their disobedience. (link)
The bolded sentence in the original Latin was:
Postulavit autem ratio, ut ista communicatio non fieret citra justitiam, quare legem dedit istis creaturis fuis, in cuius obedientia conditionem posuit, sub qua communicatio ista sieret
Instead of “should be made contrary to justice” it should be rendered “should not be made without regard to justice”. The idea isn’t injustice, nor is it mercy. Rather, God desired to declare His justice in sharing Himself with men and angels.