Does Macbeth just deceive Duncan by acting like an innocent flower while being the serpent under it?I've read the end of Act 1, beginning of Act 2 numerous of times but I think I'm missing something.
You're not missing anything, you've just not read far enough into things. Of course, the MacBeths deceive King Duncan by inviting him to their castle and planning regicide.They pretend to be nice like "flowers" and they'll be the serpents who will sting the king and eventually kill him. It's not "just" deceiving him however, because to kill a King is to set into motion a string of "unnatural" events. MacBeth is struggling with the act and their plans. Lady MacBeth is trying to convince her husband to stay the course since in (1.7) he tells her "we will proceed no more in this business." He is wavering and she is steadfast in her purpose. She tells him she'd rather pluck out a nursing infant from her breast and dash it on the ground before backing out of this business and losing courage. The image you mention is a very important theme throughout the play--of pretending to be something you're not, or the recurrent motif of "loose fitting clothes."