Answer:
He was still upset because of the prophesy that Banquo's children could become king. This He found as a threat and enaged three men to eliminate them but Banquo' s son Fleance escaped murder.
Step-by-step explanation:
In act 3 scene 3, The murderers kill Banquo who dies and told his son to flee so as to avenge his death. The prophesy revealed to Macbeth that He cannot be king although his children will be kings and that Banquo's son would become king. the meaning of the statement by macbeth " we have scorched the snake not killed it" is that they still have threats to face even though Duncan is not in the picture anymore.
Macbeths laments because He recognized banquo as threat to his kingship because of the prophesy of the witch which predicted that Banquos children would become king. He is upset because of his desire to kill an innocent individual in order to maintain his kingship position.