[spoiler]Sayori said she always did have it, but Monika made it more extreme, whether it was intentional for Sayori to kill herself or not. I personally think it wasn't intentional, because Monika even says she considered the 3 other girls to be her good friends.[/spoiler]

As a person who had depression and still has a touch of it, I can honestly say that she probably was depressed from the very start. I could tell because she would focus on everyone else before herself more and seemed to not care about her own well being at all. Very clear sign since I'm like that.

You as a player don't know. It's written in such a way that a person not aware of depression in others would see things. And the only time we get to see said depression is when the writer of the game wanted us to see it. We were first given a hint about this when she started being distant at the school. And that was around the time you as the character started becoming much closer to everyone. You could argue that's when Monkia could have edited Sayori's files to have depression. And you could also say that Monkia coached her along further down depression because they were always talking and hanging out. Even before the depression. You could even say before all of that Monkia found out that sayori liked you and thus she started to tinker with her files. When Sayori says something like "Monika was right I should.." or something like that It could also imply several things. Like "he is totally into you. Talk to him" causing the situation where you confess feelings or not. Or she could have even pushed her to commit suicide saying something like "It's only going to get worse. You know how he feels. Just end it and save yourself and him the pain." But instead of killing herself right then she went out and saw you. She couldn't help it. You were her best friend. Thus forcing the situation where you confess your feelings or not to happen anyway. I think the writter personally left things open so it can be seen from different angles. Monkia can easily be written as a bad person because of the bad things she did. But if you actually listen to her and stick around for awhile after the game is near it's end you can see she's not the generic evil person that the game first throws in your face.

Monika basically took all their bad qualities and made them worst

She always had it. Monika just made it worse

Monika gave her depression/made it worse