Sorry, can someone please explain the ending??

I think she did it partialy out of vengence for him trying to "kill" the original Somebody. He kept all of her human friends alive, but he crossed a line when he tried to destroy her creation.

When she viewed Somebody's p.o.v. video footage of Oh bashing it, you could see that she did not forgive him. Her co-founder had it right when she said that Somebody was Sum's everything. And at the end after Sum completed the deed and took off the swethshirt and latex gloves, you can see "Somebody" tattooed on her finger.

Another way to look at it is that she wanted to put him out of his misery, just like how she killed the cat in the first episode.

My theory is that she is the main villain. she made this website only to find and kill such men. But this time, she believed he loves her (though he actually loved her AI) that's why she slit his eyes because she never wanted him to know that it was her who killed him.