There are several possible interpretations for those last two lines. One possible option is that the speaker's wrath killed the other man. To clarify, as the speaker of the poem kept his wrath within him instead of speaking about it, it grew and grew. It became more and more of a burden, no matter what he did, whether it be that he "waterd it in fears," or "sunned it with smiles." The wrath becomes so large and noticable that Blake uses a metaphor of an apple growing from a tree to describe how it feels to him. Then, at the end, he states that "In the morning glad I see;/My foe outstretched beneath the tree," which seems to indicate that the foe has partaken of this poisoned apple, and has died. That is one interpretation. If your wrath and anger for someone is nurtured within you for so long, it eventually turns to poison, and that poison often does great damage to the person you are angry with. It's kind-of like bottling up rage and having it explode, with disasterous and regretful results, instead of just dealing with your anger right away. Blake uses a poisoned apple as a metaphor for that toxic anger that the reader grew by dwelling on it, and his foe, outstretched on the ground, is a symbol for the speaker having released his anger finally, and how it completely destroyed the other person. I doubt Blake is referring to actual murder, but think of the consequences of blowing up at someone; it ruins friendships, self-esteem, families, marriages, and often has lasting impacts. Blake's moral is that when we are angry, we should, as he did in line 2, talk about it and get it out there, so that it can "end" in a better way.
I included ther possible interpretations to the last two lines in the link below, take a look at those too and see if they help. Good luck!