2018 Game 5 vs OKC: Blew a 25 point lead, lost
2018 Game 6 vs OKC: Blew a 13 point 4th quarter lead, got bailed out by the refs
2020 Game 5 vs DEN: Blew a 15 point 3rd quarter lead, lost
2021 Game 2 vs MEM: Blew a 3rd quarter 21 point lead, still won
2021 Games 3 and 4 vs MEM: Blew double digit 4th quarter leads, still won
2021 Game 2 vs LAC: Blew a 21 point lead, still won
2021 Game 6 vs LAC: LMAO
If you still don't think lack of killer instinct/mental toughness is a problem with this team, your head is in the sand.