Can a 12 year old be a bad person?

I ask this because I started thinking about those two 12 year old girls who stabbed their friend 19 times, and left her for dead to impress slender man or something. The girl ended up living. I guess one girl was diagnosed with schizophrenia and has to spend 40 years in a mental hospital. The other one I believe was sentenced to 25 years, but she got out early and will be monitored until she's 37. I don't know, I feel like when I was 12. My mom said that their sentences are too harsh, and she says that they aren't bad because they were just kids when it happened. What do you think?

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Typo. I meant to say that when I was 12, I knew that stabbing someone was wrong.
Can a 12 year old be a bad person?
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