Before our speaker eats the plums, he knows he shouldn't, that someone else is saving them for a delicious breakfast they'll never get to have. So, the entire poem is a note of apology, in which the speaker asks for apology and expresses his guilt. Even the line breaks of "This Is Just To Say" can be read as an expression of halting writing; the guilty guy is struggling to find just the right words to fess up to his plum crime.
The poem's short line lengths show that the speaker is hesitating, because he feels guilty.
This guy doesn't feel guilty at all. Otherwise he wouldn't have eaten the plums in the first place. No, this dude is just gloating.