Learn English – What’s it called when you unfairly wrong someone just because you don’t like them

This is a Recrimination:

an angry statement in which you accuse or criticize a person who has accused or criticized you

As Tonepoet mentions in the comments below, one who recriminates is a recriminator. While the definition of recriminator perfectly fits the person you describe, the word's rarity makes me hesitate to recommend making use of it (recrimination is more common).

A step below a junior position makes me think of an "undergraduate trainee" or a "student trainee" as mentioned by the O.P.

• trainee - one that is being trained especially for a job MW
• a person undergoing training. TFD

"a trainee manager", "a trainee teacher", "a trainee solicitor"

job resposibilities of a trainee engineer

• The trainee engineer is majorly responsible of the tasks that are desk job nature.
• The engineer trainee is responsible for the designs of the commodity depending on the sector in which he/ she is based.
• The engineer trainee is responsible for managing the data and all the information related to the particular project he/ she is assigned to.
• The trainee is responsible to follow all the directions given by his/ her mentor.
• The trainee engineer is responsible for completion of all the assigned tasks in the given deadline. All tasks assigned are for the trainees own learning.
• The trainee is responsible to carefully perform the tasks he/ she has been assigned and keep the mentor updated about the progress of the task/ project.
• The trainee is responsible for reporting to his/ her mentor after the completion of each and every task. And present to the mentor a summary of the project.
• The engineer trainee is responsible for preparing a report in how the training has helped him/ her in understanding the dos and don’ts of the sector. The report should be a comprehensive representation of his/ her experience.

To make sure it means someone who is still a student, the word "undergraduate" must be included.