The picture is not clear so one can't really tell if it was a gun he was holding like that or not but the police can use some technology to make it more clear or dictate if that was a gun he was holding, if they do that and find out it was a gun he was holding then he should be arrested, and interrogated to find out if he was really the one that shot at Charlie Kirk because it may be a gun his holding but it could be for self defense as long as he is license to hold that gun when going out then one can't accuse him for the murder of Charlie Kirk.
The crime should be investigated very well so as to fine out who really killed Charlie Kirk, I believe there should be CCTV cameras in that place he was shot and people in that place may know where the gun shot came from, I believe if the police and FBI dig into that matter they will find the curprit.