"I don't necessarily write everything as automatically assuming it will be collected, there's nothing that says Hitman will be collected, though it might be"

Garth Ennis is suggesting that he does not write with the expectation that his work will be gathered into a single volume. He is indicating that he does not presume that his work, such as Hitman, will be gathered, but that it is possible. This suggests that Ennis is open to the concept of his work being gathered, but he is not counting on it. He is likewise recommending that he does not write with the expectation that his work will be gathered, but rather he composes for the satisfaction of the reader. This quote reflects Ennis' mindset towards his work, which is that he is open to the idea of it being collected, but he does not anticipate it to be. He is content to let his work stand on its own, and he is not extremely interested in the idea of it being collected.