‘But Jesus Iscariot, one of his disciples, the one who would betray him, said, “Why wasn't this perfume sold for 300 denarii, and given to the poor?” He said this, not because he cared for the poor, but because he was a thief, and as he held the communal purse he used to take money from it'.
It is natural for a Christian to deplore waste, but here we have a reminder that some things which at first appear wasteful, are rather of great value. Mary's expression of love would have lost its meaning if the oil had not been valuable. ‘300 denarii' was almost a year's wages for the average worker.
Into that fragrant atmosphere came muttered voices of dissension. Mark tells us that Judas was not the only one who murmured. It is clear that Jesus' teaching on the wise use of possessions had sunk in, but as can often happen, a certain hardness had also crept into the thinking of some of the disciples. Jesus would remind them that a balance needs to be struck. Their hardness contrasted with Mary's generosity of spirit.
In one sense both were right. The general principle is a wise use of possessions, but there are occasions when an exceptional situation justifies extravagance. (However this does not justify extravagance in general, nor the spending on huge, ornate cathedrals and expensive vestments, as suggested by some. Those who truly give of themselves in the building of such things, because of their love for Christ, do well, but such buildings and accoutrements are usually for man's glory rather than God's. The motive for them is often as unlike Mary's as can be).
John, however, draws out that Judas, the treasurer of the group, was the one who voiced their grievance as they muttered together. He is also aware that Judas was especially guilty, for unlike some of the others, Judas had a deeper motive for his grievance. He wanted a full purse so that he would be able to dip into it more easily without being found out. Possibly John had reason to know something that was not common knowledge, even though the final proof was lacking. John is contrasting the mind of one who loved Jesus with purity and overflowing generosity, and wanted to give Him everything, with the mind of one who had become so consumed with a love of money that it would finally destroy both him and Jesus.