By dark traits I'm going to assume you mean traits that are a challenge to conquer and create problems within the relationship.
I will say the “darkest” trait of Aries would be the temperament. An Aries is quick to anger, and once they're mad it's close to impossible to talk them down. Being ruled by Mars, they are always ready for war, be it verbal or non verbal. Aries can say and do things in this state that will bear tough consequences, and Aries will still be too stubborn to regret their actions.
Another dark trait would be their promiscuousness, they are quick to spark a flame and even quicker to enjoy it. They will enjoy many casual relationships before finding one to settle for, and even still will maintain their flirtiousness that they disguise as friendliness. 😉
So there you have it. Two “dark” qualities of Aries. Hopefully with this information you can control your impulses and learn to think before you act but…. Thats a conversation for another day.