Saw this somewhere recently:

Zeal is not justified by its object. Even a cursory glance at church history tells us that many of our theological heroes, who were honestly and in good faith dedicated to the cause of the gospel nonetheless allowed their passionate commitment to Christ to spill over into metaphorical and sometimes literal bloodshed: John Chrysostom (the 'golden-mouthed' preacher who was obscenely antisemitic), Martin Luther, John Calvin, to name but a few...

No one could deny their 'zeal for the Lord.' They were deeply 'concerned for Gospel fidelity', to use your own phrase. But it nonetheless led them down all sorts of regrettable paths, that the church has paid for ever since.

In other words, we can justify intolerance and vilification of those who do not believe the things we do, simply by saying that we are 'zealous for Jesus.' The fact that Jesus might be the object of our zeal does not in any sense make our resultant hostility any the better.

I'm reminded of a certain 'Yes, Prime Minister' sketch, which could be paraphrased along the following lines:

"Zeal is one of those irregular verbs. 'I am zealous, you are passionate, he is a rabid fanatic.'"

At times, whether our zeal is godly or not might be all in the eye of the beholder...