It's a more subtle kind of evil too. Not an act of willful malice (though it may turn out to have involved such later), but of apathy towards plight, particularly as affecting those seen to 'deserve' it. There was likely reluctance to the plan from the outset - helping the Federation's oldest enemy and such - but when Admiral Jean Luc Picard tells you something is the right thing to do, well, kinda hard to argue.
Then, tragedy strikes. A terrible, if seemingly unrelated, tragedy. A tragedy that has cost tens of thousands of lives. Undone all the effort poured into the endeavour. That burned to the ground not only a major tactical asset, but also Earth's sister world. The symbolic value of that alone would drive up people's sheer incense towards doing anything further. Look at what it's cost us, why should we care about them?
That is the little devil that waits in the shadow for a society like the Federation.
