25 Judas, who betrayed him, said, “Is it I, Master?” He said to him, “You have said so."
27 Then after the morsel, Satan entered into him. Jesus said to him, “What you are going to do, do quickly.”
According to the commentary in my Navarre New Testament:
The morsel (at the last supper) Jesus offers Judas is a sign of friendship, an invitation to him to give up his evil plotting. Judas rejects the chance he is offered. St Augustine says what he received was good but he received it to his own perdition because he was evil. Satan entering him means that from that moment Judas gave in completely to the Devil’s temptation…
…These details have been shared so we will bare no ill will against those who wrong us. Christ washed Judas feet, reproved him without bitterness, ministered to him, allowed him to share his table and his kiss. Judas did not become better but Jesus persevered in his course of action