Picard's line made him seem hopelessly naïve; with Q's power, he could have used his limitless ways of warning the Federation about the Borg, even transport one Enterprise member to the parts of the galaxy where the Borg had assimilated and/or destroyed civilizations. That would be warning enough, but he was being vindictive and did not care that an unprepared Federation--all of those lives/worlds--could have been wiped from history through assimilation.
I don't deny any of that. I was just saying Q was a complicated being. He could be twisted, dangerous and vindictive. But at the same time I don't think he wanted to see humanity destroyed. As time went on he did seem to gain a grudging respect for humanity.
I think in some ways that's what made him such a great character. He wasn't really a villain, or a hero, but in fact he could be all those things at the same time.
In "Q, Who" he was indeed being vindictive. He didn't really care 18 lives were lost. But at the same time he gave Picard, and the Federation, a vital warning of an extremely dangerous villain that sooner or later was going to come calling. The Borg were coming. But Q's actions gave the Federation an early warning, a heads up. So while being vindictive, he was also giving a helping hand. And he actually seemed to gain a new found respect for Picard when Picard basically begged Q for his help.