I've read both separately:
After Macbeth turns King, he loses it and can't handle the person he's become (for example he hallucinates showing his weak mental state). The reason he became King was because he killed the previous King. This was bad because... This showed weakness in his ambition because...
In Of Mice and Men, Geroge and Lennie travel together, but previously, they had been "ran outta weed" because of Lennie's incapabilities. This shows flawed ambition from the start because...
There, there's the start of your essay.