Wa ‘alaykum as-salaam wa rahmatUllaahi wa barakaatuh,
1) Allaahu a’lam. He is probably refering to major sins and intentional “minor” sins, wAllaahu a’alam.
2) Just as someone would be ashamed to commit a sin in front of someone else, then likewise we should be ashamed to commit sins in front of the two angels. Even if the one on the right only writes our good deeds, he still see’s our sins, and from haya is to feel ashamed in front of others.