We also live in a time when the CIA and FBI has been trying to remove a duly elected POTUS for the past three years. Why? because they think they know better than the unwashed masses.
What do you do when in your opinion (possibly supported by classified evidence) that duly elected president is compromised by foreign intelligence? What do you do when the president makes decisions that hurt the US and help countries opposed to us?
Suppose there's some irrefutable classified evidence that he's compromised and under foreign control. Do you really sit by and do nothing?
Classification isn't holy writ. In order to save the union, you'd be justified to publish anything under your own name. That would be more believable than anonymous leaks that can't be discerned about the general noise floor of anonymous leaking. Snowden put his own name on his releases. Why can't this hypothetical heroic spook do the same?
That doesn't absolve trump of consequences for his own actions though. If he's dirty too, then he needs to go.
When it was declassified, it turned out that Nunes was lying. See https://vault.fbi.gov/d1-release/d1-release/view footnote at the bottom of page 15, top of page 16. Steele is source #1.