Is being radically anti-Trump not a legitimate position?
Not when it drives you to apply inconsistent standards with regards to something as serious as impeachment.
Was Liz Cheney always radically anti-Trump?
She is a representative of the establishment Republicans (as evidenced by the fact that they put her in the third spot in the house in her second term) which is radically anti-Trump. I don't know what she personally said about Trump.
At least we can presumably agree the impeachment isn't a crazy Democrat agenda...
Trump was elected as a repudiation of the establishment Republicans. When those people vote to impeach him, it's not people on "his side" crossing over