Real Betis played well in the first half and got the goal. After the break, I don't know if the idea was to hold onto the lead, and if that was the case, it was a dumb idea. They shopped pushing for goals, and they paid dearly for it. Chelsea on the other hand, was nothing but brilliant. They kept fighting and took their chances. After the first goal, Betis couldn't stop them anymore and the goals just kept pouring in.
Real Betis played very well especially in the first half just like you said and I commend them for that. Betis thought they had already won the game when they were 1-0 up and felt too comfortable forgetting that the game of football is not yet over until the final whistle. I initially doubt Chelsea but they proved me wrong with yesterday performance and the incredible come back.
On the other hand, Chelsea need to up their game especially in the attack because I almost stop watching the game seeing how much precious opportunities were being wasted. They will be playing in the champions league next season and they won't succeed in the earliest round with this performance unless they make some changes positively.