So regarding that lap Checo was on in Monaco before he binned it, I watched it again this morning on the live timing view to watch his and the others' sectors. Checo improved in the first sector (couple of purple mini sectors) but did not get an outright purple first sector. His second sector was all yellows up to his crash. Le Clerc set a purple first sector, IIRC.
So Checo wasn't improving and he knew it. He also probably gathered that Max was going to push him down further - but Max almost always out qualifies him anyways. So, as has been mentioned, would he bin it to lock in 3rd with all the associated mechanical and FIA / Team risk associated with something so unsportsmanlike? Very difficult to say and much more on the unlikely side than being a solid conclusion.