It bears repeating, I don't dispute that the CE likely needed destroying. The dispute is in this "kill first, ask questions later" attitude certain people are employing. I will never agree to that kind of morality. Such deadly force should be used if and only if other avenues have been explored, to the extent such can be done safely.And look where that got him.
The simple fact of the matter is that the Enterprise needed to destroy the CE. If they didn't then they would be responsible for everyone else it would kill afterwards and all the worlds it would strip mine. This creature served no purpose other than galactic destruction as it required to do so to continue to exist.
(Which, and I want to be absolutely clear on this, specifically allows for a life-and-death situation when such force must be used, because a less violent solution is not available without putting lives at risk).
In the specific case we're talking about, there was a possibility being explored that had potential to yield a non-violent solution, without undue risk to the crew. If such could not be found, Picard would have taken the CE out himself. He did not recieve that chance. Dr. Marr was very much in the wrong. To me, it's unambiguous.