Answer:
To add suspense and keep readers engaged
1. Macbeth's second and third victims are the two guards who were standing in front of Duncan's bedroom door. His plan was to kill Duncan in his sleep, but the guards were preventing him from doing that. This is why he killed the two guards with the help of his wife. The reason why he did that was so that he could have someone to blame for the death of the king - he would say that he caught them killing the king, which is why he had to have them murdered as well.
2. Lady Macbeth fainted to distract everyone's attention to her. She knew that Macbeth was weak, and that if questioned, he would admit to everything - to killing Banquo, Duncan's guards, and Duncan himself. This is why she decided to take matters into her own hands and therefore she pretended to faint. Thus everyone gathered around her to help her, and Macbeth 'fortunately' didn't get the chance to reveal their crime to everyone.
