Verm yes he was involved but he didnt order to have the guy killed or kill the guy himself. The News Media leaves that impression on folks.
He has been found guilty and has multiple life sentences. Turn him over to the Italians and that should be that.
I believe the Egyptians (could be wrong as I dont recall off hand) gave the Americans tapes of intercepted phone calls from Abbas to the hijackers. They wanted more evidence to prove his involvement in the actual murder. If anything the tapes would have aided in his defense. However, hes a terrorist and as such he earned those multiple life sentences. There should be no "trying to figure out what to do with him".