The Hogwarts house people love to hate in the Harry Potter books. (It doesn't help that Slytherin was constantly being painted as the villains in the story.)
Known for being cunning, success seeking, loyal to a fault, but also ambitious.
In truth there are a lot of really good people who are Slytherin, as they do have good traits. Ambition is a good trait to have, but I've always felt ambition taken to the extreme is never good. (Though most things taken to the extreme isn't good.)
This little fortune cookie highlights where I feel ambition can go wrong:
"Ambition knows no obstacles."
True, if one is truly ambitious they are ready and willing to weather any storm. They are ready and willing to face and overcome any obstacle. This is a good thing, but when these obstacles become people then it's good to pause and reflect. I'm not talking about overcoming naysayers. I'm talking about a persons willingness to hurt other people and/or stab others in the back to achieve what they wish.
No ambition is worth hurting another.