1. Suppression: The speaker initially tries to "hide" his anger from his foe, pretending to be happy and kind. This suppression of anger is represented by the act of watering the anger "with cold disdain" and allowing it to grow "in secrecy."
2. Expression (but in a destructive way): Ultimately, the anger cannot be contained, and it manifests as a poisonous tree. This represents the speaker's choice to express his anger, but in a destructive and vindictive way. He lets his anger fester, poisoning his own soul and ultimately leading to harm for his foe.
The poem argues that suppressing anger is unhealthy and ultimately leads to its eruption in a more harmful way.