My take is he said it to partly see mc’s reaction also partly to show indignation for being left alone. It could also signify he no longer cares what happened in the past and want to look toward the future rather than past.
Also that diary had such a painful last words of the ml
that's assuming he knows mc would definitely see it in the bin which isn't certain. at that point they were also happy with each other so why would he try and hurt the mc by making him feel guilty for leaving? and the whole manhwa is about memories and he throws those memories away? i definitely see your point in throwing the diary away bc of how sad the last sentences are but idk about ring
idk about the ring being thrown away as well*. sure we could just say it brings sad memories back but it also hold so many happy ones which are the focal point of the manhwa.
im rereading this and i still am not exactly sure why he trashed the diary along with the ring? he regained his memories when coming into contact with it so i would imagine he might keep it at least for sentimental reasons, but he went and said that "why keep that ring when there's nobody to give it to". he's not a manipulative person so i don't see a reason why he would've said that to hakyung either? it's not like he wanted to guilt trip him or something.