After doing his "betrayer" job, was there no chance of forgiveness from God for Judas? Based on Isaiah 1:18, could Judas be a sinner who repented, and then committed suicide?
Our understanding on any matter in Scripture must be based on what Scripture itself says. Proper interpretation of the Bible does not depend on speculation. In the case of Judas, the Bible is consistent in portraying Judas as an unbeliever. For example:
John 6:70 Jesus answered them, “Did I Myself not choose you, the twelve, and yet one of you is a devil?”
John 6:71 Now He meant Judas the son of Simon Iscariot, for he, one of the twelve, was going to betray Him.
As Jesus indicated when He dismissed Judas from the meal, He required that one of His apostles be an unbeliever so that someone would betray Christ in the end. True believers in Jesus wouldn’t have betrayed the Lord in that way, yet it was necessary that Christ die at the hands of a betrayer. So the Lord chose to include Judas in His inner circle of disciples despite being without faith. Later, we’re told Judas was indwelled by Satan himself:
John 13:26 Jesus then answered, “That is the one for whom I shall dip the morsel and give it to him.” So when He had dipped the morsel, He took and gave it to Judas, the son of Simon Iscariot.
John 13:27 After the morsel, Satan then entered into him. Therefore Jesus said to him, “What you do, do quickly.”
The Bible testifies that a believer in Jesus Christ cannot be indwelled by the enemy, since that person has been sealed by God. So had Judas been believing in Jesus, Satan would not have had the ability to dwell in Judas. Later, after Judas betrayed Jesus, he committed suicide, but his death was not a sign of repentance. It was the final act of Satan to free himself from Judas’ body.
As Pastor Armstrong explained in his recent teaching in Matthew (Lesson 8F), demons cannot leave a body they occupy of their own accord. They must either be cast out by God or else they must wait for the host to die. Only after the host body has died is the evil spirit released to roam again. Consequently, demons commonly drive their hosts to commit suicide in order to release themselves from the body. That is what Satan did to Judas, so his death is not a sign of repentance or faith.
Furthermore, suicide is never the natural response to new faith. On the contrary, a believer in Jesus Christ would be motivated to honor Christ with a life of service. Consider the example of Paul, who persecuted the church but once he came to faith, he became a great evangelist for Christ. This is the natural response to faith, and Judas’ suicide is evidence he never believed. Rather, he was an unbeliever until death, and he has suffered a great punishment as a result:
Mark 14:21 “For the Son of Man is to go just as it is written of Him; but woe to that man by whom the Son of Man is betrayed! It would have been good for that man if he had not been born.”
Finally, Isaiah 1:18 was not written about Judas. It describes God’s saving grace extended to the believer, so that verse has nothing to tell us about Judas’ situation. Therefore, since Scripture never indicates that Judas was a believer, and on the contrary, every reference to Judas in the Bible indicates he was an unbeliever, we cannot suggest or assume that he was saved in the end.