"Well... no," Harry said. "Not lately, anyway. If I say that, he'll take that to mean that he should take those kinds of things into his own hands because I'm 'not ready' for them."
That was the real problem. Any ethical argument Harry would make would just cement the idea in Ron's mind that Harry wasn't ready for how low they would have to go to win the war.
Then he realized something, and looked at Ginny. "Do you think it's wrong, what we're doing? Last time I saw you..." He trailed off. The last time, she'd said that Voldemort had been doing everything for the good of the Ministry all along, and she didn't understand why Harry was still fighting. He'd half-expected her to say something to that effect, but she hadn't.