Let it just be eugene!! Please dont let it be Rick, Carl, Daryl, Abraham, Glen, Michonne, Maggie, sasha or any other good character
From the lighting on Negan's face it's one of the 4 on the left (Eugene, Carl, Spencer...). Taking in account you can't see the RV, it can only be the 2 most left (Eugene and Carl). As Negan says "If anyone does anything; take out the boys other eye" you can assume it's not him since that threat would be meaningless if it was Carl. Only one is left based on position and the dialogue. It is Eugene
In my opinion, it's Glenn. They have been setting the atmosphere for Glenn to be killed off time after time so now that it actually come down to it, we shouldn't hurt as much... well DAMMIT! IM HURT! So not ready for Glenn to leave the show.
If you break down the scene, behind nigan are some trees, no two are alike. Trees are a source of fingerprints basically...if u line them up to negan as he leaves his trailer in the background to the seconds before he swings his bat, then u see its the same grouping....so it is my fault he killed the guy that got daryl his bike for recruiting....it was Aaron! Thats my thought.