If you get into the position Bettinelli is in and try and shift your weight from a neutral position to you left foot. The first thing you'll notice is the body shift towards that side, your knee and shoulders will dip slightly (not a lot but slightly) towards the side you've shifted your weight to.
You see that movement from Bettinelli after the ball is past Spence, not before the ball is hit or the moment it is hit. It is my opinion that Bettinelli gets caught flat footed and reacts too late.
Is he the sole factor in the goal being conceded? No.
Is it a difficult save to make? Yes.
Should he do better? Yes.
This is largely on Klopp, the man doesn't have a Plan B, he runs the same 15 man squad into the ground and doesn't drop players if he can help it. Injuries to key players haven't helped him of course but that just highlights how Liverpool don't have the strength in depth the way the likes of Man City, Chelsea and Man Utd have. Again, that's largely down to Klopp.
It happened at Dortmund and Mainz as well.