in my eyes he becomes obsessed with objects e.g the flower. he didn’t like that the caterpillar was “obstructing” his ability to possess the flower so he kills it. this way, the flower is his and only his. the same with the fountain pen. he wants THAT fountain pen because he fixated on it. he’s an intelligent psychopath so we see him decide to manipulate his sister into using a diary to write down her darkest thoughts. it would act as the perfect evidence for suicide, so that he could kill her without being found out and have the fountain pen. in his eyes, she was the bug.
now we see this happening again, but for the first time (it seems), with a living person. he must have thought that her crying was pretty or smth idk his obsession lacks real justification. he now wants to possess her, so he will get rid of any and all obstacles that would prevent him from having her. this is a different case too, since he will also want to make her depend on him and only him at any costs.
maybe it’s too early to presume but i have a feeling that it’s not about the objects that he feels attracted to; it’s the misery of the people who own them. first case with the flower. he doesn’t technically need to kill the caterpillar, but he does it for the kick of it. another example is the sister. while the suicide note does become beneficial to him to sell that his sister was really suicidal, i think his main goal was just to see her in the utmost misery. and to celebrate and reminisce of his ‘victories’ maybe he keeps the victims’ objects as a keepsake? honestly rn im just focused on the fine shyts