In panel 2, Roy is having something of an emotional outburst, which he immediately apologizes for. He is in a calmer state of mind in panel 5, thinking more carefully about his words.
Indeed, it is not a grammar rule that prepositions are not things with which one ought to end sentences. It is, however, seen by many as "more formal" to avoid doing so.
I think it is a deliberate bit of characterization.