What is ironic about the expression on the lanky boy's face as he defaces Red's trunk?

As the character referred to in the text as the "lanky boy" (37) hunches over near a stop sign down the street, Red watches. Red notices that the boy has a tool of some kind in his hand and that he is smiling "just a bit" (37) as if he has just told himself a funny joke. The irony, of course, is that in just a moment, the reader will find out that the boy's intentions are nefarious and hateful, rather than carefree and humorous.