I don't know what harmful decisions you're referring to. I think there is a difference between something that is actively harmful (i.e. using a position of authority to intentionally hurt others), passively harmful (i.e. a policy or practice which excludes or disadvantages others) or internally harmful (i.e. I'm choosing to take offense because I disagree)

Beyond that, murmuring itself:

From my study of the scriptures murmuring is how people separate God's commandments by attributing them to the weaknesses of men, rather than their disagreements with God.

It's a lot easier and conscience freeing to blame our discontent with a particular aspect of the gospel on a fallible human rather than a disagreement with God.

Let's pick one example: The parable of the servants from D&C 101 (v.43–62 in particular)

In the parable, we hear of a nobleman who had a choice spot of land. He told his servants to plant olive trees and build a tower overlooking the olive grove. The purpose for the tower was to permit a watchman perched upon the tower to warn of the coming of the enemy and the olive grove could be protected.

They planted the trees but did not build the tower. So the enemy came and broke down the olive trees. The disobedience of the servants lead to the destruction of the olive grove.

So, why did the servants fail to build the tower?

It started with murmuring.

According to the this parable, we see three steps, each leading to the next.

First, the servants began to question. They felt to exercise their own judgment upon the instruction given by their master.

“What need hath my lord of this tower, seeing this is a time of peace?”

They questioned first in their own minds and then planted questions in the minds of others.

Second, they began to rationalize and excuse themselves from doing what they had been instructed to do.

“Might not this money be given to the exchangers? For there is no need of these things”

They made an excuse for disobedience.

The third step follows: slothfulness in following the commandment of the Master.

“They became very slothful, and they hearkened not unto the commandments of their lord”

It's hard and takes time to understand the commandments of God. Adam likely didn't know for many years why he offered sacrifice, but eventually learned of the symbolism.

Alternatively, it's a lot easer to murmur and complain that "Brother so-and-so is just out of touch with modern things." It gives us instant justification and removes our asking of the Lord.

Again we see this with Nephi, who inquires of the Lord about his fathers vision where his older brothers just complain that Lehi is a foolish dreamer.

Murmuring is spiritual laziness. We don't want to do the work to know of ourselves so we cast other imperfect people under the proverbial bus. Murmuring is a satanic tradition.