Wow, this is a great question, one that's hard to answer. Regarding Pius Thicknesse, it's possible that he was not driven by blood purity laws or beliefs. In Deathly Hallows, Yaxley says this about Thicknesse:
Once again, all heads turned. Yaxley squared his shoulders. ‘My Lord, I have good news on that score. I have – with difficulty, and after great effort – succeeded in placing an Imperius Curse upon Pius Thicknesse.’
Deathly Hallows - page 12 - Bloomsbury - chapter one, The Dark Lord Ascending
On the next page, Voldemort says of Thicknesse:
‘As long as our friend Thicknesse is not discovered before he has converted the rest,’ said Voldemort
Deathly Hallows - page 13 - Bloomsbury - chapter one, The Dark Lord Ascending
Thicknesse was Head of the Department of Magical Law Enforcement when he was placed under the Imperius Curse. The fact that he had to be placed under the Imperius Curse at all indicates that he was not a blood purist. Yaxley notes that it was extremely difficult to curse Thicknesse to begin with. When Barty Crouch Jr as Mad-Eye Moody teaches the fourth years the Imperius Curse in Goblet of Fire, he reiterates that the Imperius Curse can be resisted. So to put the Head of Magical Law Enforcement under the Imperius Curse must have been a tremendous feat. It has to do with the reason behind the Imperius Curse, i.e. why the Death Eaters wanted to control him.
It's clear the Death Eaters wanted Thicknesse to help turn other Ministry officials (and whoever else) to their agenda, which included blood purity. That they needed to use the Imperius Curse at all indicates that Thicknesse was not a blood purist prior to begin with.
What's not clear is when, or if, the Imperius Curse was lifted. Thicknesse fights alongside the Death Eaters at the Battle of Hogwarts -- does this mean he's still under the control of the Death Eaters, or does it mean he's been on the Death Eaters' side all along?
I say he was under the Imperius Curse when he said the quote you've highlighted in your question. That was the whole point of putting him under Imperius to begin with: 1) He was needed to convert non-believers to the blood purity line of thinking and 2) Voldemort needed a puppet to cover the fact that he was indeed controlling the Ministry and politically speaking, it makes sense that the Head of the Department of Magical Law Enforcement would be poised to become the next Minister for Magic -- as Barty Crouch Sr was set to do before his son was outed as a Death Eater -- which may be why the Death Eaters specifically wanted Thicknesse under their control.
No, Thicknesse was likely not a blood purist when he was placed under the Imperius Curse.