We know that Paul and Peter both repented, and Judas' guilt drove him to suicide (we're not told Judas' eternal destination).
Matthew 26:24 "The Son of man goeth as it is written of him:
but woe unto that man by whom the Son of man is betrayed! it had been good for that man if he had not been born."
Jesus probably wouldn't be saying such things about someone who was going to Heaven.
Does that gurantee them a spot in hell? Does it undo all the other times they have done good?
Presumptuous sin(when you know you're doing wrong and do it anyway) is never a good idea.
The Bible says in Ezekiel 33:13 "13 When I shall say to the righteous, that he shall surely live; i
f he trust to his own righteousness, and commit iniquity, all his righteousnesses shall not be remembered;
but for his iniquity that he hath committed, he shall die for it."
It also says that unforgiveness will cause you to not be forgiven by God. It doesn't matter what they did, God requires you to forgive them if you want to receive His forgiveness. In fact, given the parable of the unmerciful servant, if you don't forgive others, God will actually reinstate your sin debt if you refuse to forgive! In addition, harboring hatred towards someone in your heart is actually the moral equivalent of murder in the sight of God(1 John 3:15)! The only difference between the person who commits actual murder and the person who harbors hatred is that the person with hatred WOULD commit murder but doesn't since they fear the consequences. If the consequences were gone, so would any reservations against murder.
A good way to deal with your angry feelings towards whoever you're so angry at is to pray to God, honestly tell Him your feelings(He knows anyway), tell Him exactly how you feel about the other person and the things you'd probably like to do in revenge, and then turn around and ask Him for the strength to forgive that person, and then forgive them.
However, forgiveness doesn't mean that you stay around and let them use you as a "punching bag", so to speak. If possible, avoid that person physically or if they wronged you in-game(such as on WoW), put them on your ignore list and don't run instances with them anymore. If you can't avoid them(IRL), such as having a class or job with them, then pray God for the strength to not grow hateful towards them and to show them the love of Christ.
Concerning games like World of Warcraft, I can sympathize with your feelings, since I played that game before. I know how it feels to lose a roll on an item you've been grinding an instance over and over to get, I know how it feels when you see that nice titanium node only to have it ninja'd or to be beaten to it, I know how cut-throat and dog-eat-dog the game can get. As for being wronged in WoW, I would say that if that game causes you to get all bent out of shape, then delete your characters and then change the password and email to gibberish in order to render your account unusable(just in case you're then tempted to create a level 55 Death Knight).
The Bible says that Hell is so bad that if your right hand, right foot, or right eye causes you to sin that you're better to sever them and enter Heaven maimed than it is to go to Hell with a whole, full body(Matthew 5:29-30). Now, Jesus isn't promoting self mutilation, but the principle is still there. If WoW causes you to be angry, cut it off. If playing video games causes you to rage in frustration, stop playing them. A video game is not worth losing your eternal soul over.
Jesus Christ was fully man and fully God, and He understands the afflictions men go through. Jesus was in all points tempted but didn't sin(Hebrews 4:15). Jesus Christ probably got cheated at times during His life. During His time as a carpenter, He probably might not have gotten promotions or other things that He probably deserved.
In fact, in spite of all the good Jesus Christ did to the people of Israel, instead of giving Him the adoration He deserved, they yelled "CRUCIFY HIM!" When He was standing trial, no one He healed or delivered from demons or any other recipients of His miracles stood with Him. Peter denied Him 3 times, Judas betrayed Him, and the other 10 apostles scattered. If anyone had a reason to be bitter and vengeful, it was Jesus Christ. What was His response? "Father, forgive them for they know not what they do."
If He can forgive that, then He expects you to be willing to forgive people their small offenses towards you. Our sins against God are like the 10,000 talents. Others' offenses against us are like the 100 pence(KJV).
TLDR: God expects you to forgive if you want His forgiveness. Blessed are the merciful, for they shall obtain mercy(Matthew 5:7). If WoW causes you to sin, quit playing it.