Is working as an engineer for a company that supplies and maintains kitchen equipment for hotels and large restaurants permissible, or does it fall under the category of cooperating in sin and aggression?

If you are certain or highly-V of the opinion that the equipment you supply or maintain will be used for something unlawful (haram), then it is not permissible for you to maintain or supply it. If you cannot avoid unlawful acts in your job, you must leave it and search for another job; because assisting in unlawful acts is not permissible. However, if you can avoid supplying or maintaining what aids in committing unlawful acts, then there is no objection to you remaining in your job, though it is preferable to search for another job to distance yourself from places of disobedience.