Based on the poem, it would appear that what the speaker values the most is getting his own way.
He can do that with his friend by frankly confessing his anger whenever he gets upset with him. But when it comes to his enemy, the speaker only gets his way when the foe dies after eating the poisoned fruit of the tree.
One could say that what guides the speaker is a desire for power and control. That explains why his anger at his enemy only gets worse the more he keeps it bottled up. This is something he cannot control, something over which he can exert no power, and it is eating him up inside.
As a result, the speaker doesn't enjoy much in the way of quality of life. But then, this shouldn't really surprise us all that much when one considers how much he's eaten up by hate.
Even allowing for the fact that the foe's death by eating a poisoned apple finally places the speaker in a position of power, a position not unlike that of Satan in the Garden of Eden after Adam and Eve eat of the Tree of Knowledge, he is still not to be envied.
The speaker's satisfaction at the death of his enemy leaves an unpleasant taste in the mouth. Indeed, one could almost say that there's something vaguely devilish about it, in keeping with his disturbing similarities to Satan in the Garden of Eden.