Maybe Gerald saw Aaron in her room once and thought she was cheating on him. (In the previous chapter, Aaron asked Violet why she was shocked to see Aaron in her bed since he often visits her without notice. So the previous Violet allows Aaron to come and go from her bedroom.) Since Aaron and Violet were quite close ever since childhood, Gerald probably got jealous with their relationship and he probably also noticed that Aaron has been in love with Violet, though the previous Violet did not notice that.