The background story to the first verse of the Dhammapada is of an arhat killing insects accidentally, because he's blind.
The verse says,
All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.
I think we're meant to infer that if you don't act with an evil mind (or act without an evil mind), that if the killing is accidental or unintentional, that's not as bad as intentional killing.
Something similar is found in the monastic rules (page 29):
In analyzing offenses for the purpose of determining penalties, the Vibhanga divides an action into five factors: the effort, the perception under which it is made, the intention motivating it, the object at which it is aimed, and the result. In some of the rules, all five factors play a role in determining what is and is not a full offense. In others, only two, three, or four play a role. For example, under the parajika rule forbidding murder, all five factors have to be present for a full offense: The object has to be a human being, the bhikkhu has to perceive him/her as a living being, he has to have murderous intent, he has to make an effort for the person to die, and the person has to die.
If any of these factors is missing, the penalty changes. For instance, object: If the bhikkhu kills a dog, the penalty is a pacittiya (i.e. a less-severe penalty]. Perception: If he cremates a friend, thinking that the friend is dead, then even if the friend is actually alive but severely comatose, the bhikkhu incurs no penalty. Intention: If he accidentally drops a rock on a person standing below him, he incurs no penalty even if the person dies. Effort: If he sees a person fall into the river but makes no effort to save the person, he incurs no penalty even if the person drowns. Result: If he tries to kill a person, but only succeeds in injuring him, he incurs a thullaccaya [which is another less-severe penalty].