He justifies them from the debt of sin, and that is a huge motivation to shed the interests of sin, but people will still sin.
This passage and its quote show that Matthew knew early that the mission of Christ was this one Gospel, and that while recollecting what to write, he knew that all of them knew the death of Christ was going to do just that early in the ministry. It is set ups like this which bear out the truly human dilemma which we now call 'denial'. We find it in the middle of the synoptics: after confessing that Jesus is the Christ, Peter tries to 'control' Christ and tell him he can't die. Even though he knew all along, unlike the delusion of 2P2P which wants there to be another gospel in (at least) the first half of the synoptics (it doesn't work out in John at all).