The flashback with the friend shows what Taku's character was like before he fell for Haesoo - calculating, indifferent and callous - using and throwing away people - and looking at their emotions like it's something to study and explore, rather than understand. He enjoyed instigating people and watching how they'd react like the way he did initially with Haesoo by pointing him to that personal video and also how that video ended up being uploaded in the first place.... But then, that's the thing about karma... it tends to come right back to bite you...
The flashback with the friend shows what Taku's character was like before he fell for Haesoo - calculating, indifferent and callous - using and throwing away people - and looking at their emotions like it's something to study and explore, rather than understand. He enjoyed instigating people and watching how they'd react like the way he did initially with Haesoo by pointing him to that personal video and also how that video ended up being uploaded in the first place....
But then, that's the thing about karma... it tends to come right back to bite you...