You know how they say that too much of a good thing is bad? Well, it's worse when there are bad copies of something good, and downright terrible when the original thing was bad.
Let me put it this way: Ever notice how when something becomes big, people try and copy it to make a buck or two based on it? This kind of bothers me, especially when the original thing sucked.
Which brings me to Jeff OCs. (I have to tie this into Creepypasta somehow) The original Jeff story sucked. A lot. The only redeeming quality was that the picture is a bit scary (and the fact that through some Photoshop, a lot of jokes can be made; see "Let It Go To Sleep"). And when things that are bad are copied and mildly change, it's usually for the worse. (Especially when the OC interacts romantically with Jeff).
Granted, I'm sure there are good stories that can be told using the "Jeff Formula" (The formula is as follows: person is bullied, something happens, becomes psychotic killer), but for the most part, it hasn't worked.
See, I have nothing against anyone using the "Jeff Formula", just as long as they do something original and not stupid with it to make it an enjoyable story. For example, learn from the mistakes of Jeff, and try having an interesting main character. There are many problems with the Jeff story, not the least of which was that Jeff wasn't really a developed character; you didn't connect with him emotionally (unless you're a psycho fangirl), and he was more of a plot device than a character. Remember this rule: If the main character is more of a plot device than a character, then your story might suck. If you're doing the bullying/abuse angle, have the main character be somewhat likable, that way you care about the character, and feel bad that he/she is having a crappy time.
Also, make the reason the character snaps logical and understandable. Try not to make the character suffer from what I call "Plot Advancement Syndrome"; basically, something random and illogical happens that propels the plot. This can be an event or character change. For example, in the Jeff story, there was some weird feeling he had, then he went through some shit with bullies, then he snapped. That's stupid. However, after years of abuse and mistreatment, it makes some sense that one would snap. Speaking of which, this brings me to something else: "Show, don't tell". Basically, don't say, "This character got bullied and abused." Show us the bullying and abuse.
Another thing to add: if you're going with horror, don't make the only scary thing a picture. With the Smile Dog story, they built the story around the picture, and created a sense of dread and fear. While the story was based around the picture, it wasn't the only scary thing about it. With Jeff, a sense of dread or fear wasn't built up, and the picture is more of a jump scare than anything. If you're going to have a picture, follow Smile Dog's example, and build dread.
Ok, so you've done that. The character's likable, and has snapped, and is now a psychotic killer. Now, there are times when not showing something and letting your imagination fill in the details is better. See, the Jeff story would've worked better if there was less information, and just the picture. We have a scary picture, and know that he kills people. Fill in the rest. Slender Man does this quite well, as there's really little known about the character (in-canon), and most of the fear comes from how little we know of him. While I'm not saying going the Slendy route, it's just something to keep in mind if you're stumped (or lazy). If you want to go with something known, try adding something to the psycho killer concept. Sure, they might kill their victims, but what do they do afterwards? While I'm not saying that they should do something really messed up, I would recommend that you try something a bit shocking. By shocking, I mean unexpected. Maybe your character, after years of rejection and abuse, kills their victims and then keeps around their bodies as "friends" that your character will chat with. Disturbing, shocking, and a bit sad. Just an idea.
Listen, if you absolutely have to write something in the "Jeff Formula", don't settle for a cheap knockoff of a crappy character. Do something a bit different.