I've always agreed with Picard - he was willing to kill the CE if he had to but he wanted to see if he could get it to stop killing first, and let it know that it was killing sentient beings. Now if it had responded with "I don't care," then he probably would have destroyed it. But we don't know how it responded, because it was never given the chance.
Kila Marr spent a lifetime studying an entity that as far as we know was unique in the universe. The first time she met it, she destroyed it. That's destroying your own career, guys.
As for the point about the Borg, there is a huge difference between trying to communicate with a lifeform that has killed billions and letting it know that you can communicate, and stopping the crewmen for whom you are responsible from being absorbed into a collective consiousness that knows you can move and think and act of your own volition - and doesn't care. Especially if you've experienced that hell firsthand.