denigrate
To unfairly insult someone and make them seem less important.
To denigrate someone means to criticize them unfairly or speak about them in a way that makes them seem less important, capable, or valuable than they really are. When you denigrate someone, you're not offering helpful criticism: you're trying to tear them down or make others think less of them.
If a student denigrates another student's science project by calling it “baby stuff” even though it took serious work and creativity, that's denigration. If someone denigrates an entire profession by saying “those jobs don't matter,” they're dismissing the real contributions those workers make.
Denigration often reveals more about the speaker than the target. Someone who constantly denigrates others might feel insecure about their own abilities. Constructive criticism helps people improve; denigration just tears them down. When you disagree with someone or think they could do better, you can express that honestly without denigrating them or their efforts.