What's the difference between insane and mental illness?

well I know this boy from internet, he kept telling me that we want to murder some people who are like flirting around , dating with several women, just because he kept mentioned that he has no experience and no women attracted to him. and "Elliot Rodger" as his idol. I don't know much and I don't care. I often ignoring this boy's message because he's weird state. years by years he became more weird. I thought he recovered from depression like he no longer need to go to hospital. but what I noticed, he seems need to go to psychiatrist more.

same goes to my real life female friend. her behaviour more weird... I don't know how to explain. she just basically point out people are so bad right away, can't handle her anger,. something like that.
What's the difference between insane and mental illness?
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