Yeah personally, (this is just based on entirely too many hours wasted researching this case) I've always felt Jay was seriously downplaying his role before, during and after the murder.

IMO, Jay "THE CRIMINAL ELEMENT" Wilds saw an opportunity to be very badass and very edgy by playing co-conspirator to Adnan. He probably didn't think Adnan, a magnet program kid/homecoming king/goody goody, could ever actually do something next level violent and evil. Probably goaded him into it a bit, assured him he would help him get rid of the body and make sure he didn't get caught.

It was all just theoretical ("here's my car and my phone, I'll call you when she's dead!!!" "yeahhhhh ok cool...I'm gonna go smoke weed with Jen and play video games hit me up when you do it and we'll go make her disappear, just like we talked about bro lol") until Adnan called him, confirmed she was dead and the plan they concocted together had to be set into motion. He was involved, Hae was dead and Adnan had him on the hook.

As someone pointed out here, this is likely part of the reason he immediately came clean to Jen & started cooperating with police without a fight. Dude definitely found himself in some shit he did not expect would actually transpire until he was in too deep. Jay will continue to downplay it likely forever & Adnan can never confirm it either for obvious reasons as well.