In these lines, the Duke explains that in the first place, it is beneath him to talk of such matters; they are such petty and silly issues, really. He, as a duke, shouldn't have to concern himself with these things; he has more important things to do. He states, "Who'd stoop to blame /This sort of trifling?" He is upset that he would even have to speak to her of them; she should just automatically know that she is insulting him with her behavior. To speak to her is seen as being weak, and begging; it shows her that he cares. Caring is weakness. Besides, he doesn't feel like he would be able to make himself clear. He states,
"Even had you skill /In speech—(which I have not)—to make your will /Quite clear"
he doesn't feel like it would do any good. To speak to her requires skill in speaking (which he doesn't have, apparently). Then, if he were to tell her how he felt, and she listened without argument or reason for her behavior, he felt that
"—E'en then would be some stooping;/ and I choose/Never to stoop."
She shouldn't be doing these things in the first place, and to speak to her of it is to acknowledge her errant behavior; her errant behavior speaks of his lack of ability to keep her under control. It would be admitting, "Hey babe, uh, could you stop flirting so much? It makes me look bad, like you don't love or respect me." This scenario is humiliating for the duke, and very wounding to his pride. Speaking to her is not the manly thing to do; it is below his station as a duke, shows weakness, and he shouldn't even have to in the first place if she would just behave. So instead, he "gave commands" and has her killed. Yes, much better. THAT is the manly thing to do-forget a reasoned conversation...they're SO overrated. :) Anyway, I hope that helps a bit. It's an interesting poem, and a glimpse into a deeply proud and flawed man, who, instead of solving a situation with words, chooses more deadly action, to save face.