“Some won’t die before His kingdom comes?”

How can we interpret “this generation will not pass away” in Matthew 24? And what about when Jesus says in three gospels that those present with Him will not taste death until the Son of Man comes in His kingdom? Why does John 6 show Jesus saying certain things are hard to understand when He also promised life with Him would be “easy and light”? Why did God create Judas and choose him as a disciple if He knew Judas would betray Christ?