is this useful to kill the slug? If I do not mix water then will it be harmful to vegetation?

Thank you for your question. Our product hasn't been tested to kill slugs so we cannot guarantee that the product will be effective. You do not want to apply the product without mixing with water per the label's instructions. This would constitute a breach in the label's direction and is in direct violation of the EPA's registered use for the product. It also could damage the vegetation.