She had been a patient in that mental hospital for...maybe two years at that point? Feeling very guilty and crazy all that time alone.
She was just paranoid and crazy. Maybe she also just wanted to see what Michael's face looked like too before killing him. The look he had could convince her that he was her brother (so that she finally knew he would die for good). They are related, they have to look somewhat alike.
Unlikely, but, the killer could've been a copy cat killer that wanted to kill Laurie because they worshipped the idea of Myers, doing what he couldn't. See Friday the 13th Part 5.
Either way, rationally, she should've just killed him without touching the mask. I guess unveiling Michael before killing him would feel like a prize though at the same time. Like stealing his soul/identity before taking him out.