DNA and other possible small details like a fingerprint were very important details that would cause problems for her if it was left behind but the current situation was more advantageous for her.
She was not there as herself so it would not be as bad to leave such a thing behind whether unintentional or intentional.
Not only did she use the knife of Cal without wearing gloves but she released it from her hand a moment later.
She already felt the blade go deep into the body of Decker and the position was already very fatal that had been targeted.
Ash wanted to avoid a lot of the blood that came from the wound but she needed to make sure that the attack was fatal enough to ensure that the life of Decker was taken.
Decker needed to die and too much restraint would only cause more problems for her later if he was able to live on and recover.