Is it permissible to dislike someone without informing them, while keeping this feeling internal and not discussing it with others?

Not informing someone you dislike of your dislike for them is better, as it prevents animosity and discord, unless there is a benefit in informing them, such as deterring them from a sin. There is no sin in heartfelt dislike as long as it is not acted upon, and there is no accountability for what one thinks to oneself.