The whole point of the thief reference is that the parousia is a wholly unannounced event. This can be seen in following use of the thief metaphor that goes beyond the mere statement of "coming like a thief" (similarly pertaining to the manner of the parousia):
Matthew 24:42-44: "Watch therefore, for you do not know what hour your Lord is coming. But know this, that if the master of the house had known what hour the thief would come, he would have watched and not allowed his house to be broken into. Therefore you also be ready, for the Son of Man is coming at an hour you do not expect".
As for the idea that Jesus would not be associated with negative things like thieves, Jesus' parables and metaphors in the synoptic gospels commonly utilize traditionally negative imagery. The kingdom of God is likened elsewhere to leaven (an impurity) and a mustard seed (a noxious weed). Jesus holds up eunuchs and Samaritans as examples to imitate. The master in the Parable of the Shrewd Manager commends the steward's dishonesty. The Parable of the Broken Jar and the Parable of the Assassin in the Gospel of Thomas similarly use negative themes positively. This made Jesus' parables particularly striking, with their fondness of unorthodox reversals, grotesque comparisons (such as timbers in eyes and camels through needles), and violent themes (such as the Parable of the Warring King).