The correct answer is colleague.
A colleague is a person who works with another person in the same profession or occupation. In the given sentence, the speaker is saying that the person would have been a close associate of theirs if they had not been disloyal. This suggests that the two people were colleagues, as they worked in the same profession or occupation.
The other options are incorrect because they do not accurately describe the relationship between the two people in the sentence. An employee is someone who works for another person, while a competitor is someone who is trying to achieve the same goal as another person. An executive is a person who is in charge of a company or organization.
In conclusion, the correct answer is colleague.